Weed can cause anxiety in and of itself. Not just social anxiety. Physical anxiety is what I believe the problem to be, and that in turn causes the social anxiety.
You don't have to have "underlying issues" or any of that nonsense. I know the chillest people who smoke a joint and won't say a word, just freak out in their own minds so they don't touch the stuff. These are people who back in high school loved pot. If you get anxious while high but you're not when you're sober then point the finger at the chemical THC which is screwing with your brains and making you more anxious which is probably the most common side effect of the drug that is reported. Especially if you have been smoking too much, this can happen to veteran potheads and all of a sudden they can't smoke weed ever again. But are they going to quit? It's probably going to take them some time to figure it out, that there are no longer any positive effects and that the experience has been ruined by overdoing it. That's what happened to me it took forever to admit, since for the longest time before I started getting crazy anxious, I loved the herb.
Someone said people with anxiety problems get anxious when they are stoned. That's not always true. Sometimes people with genuine anxiety problems calm down when they smoke a bowl and get relaxed. Sometimes people with no anxiety issues whatsoever will go completely insane off the tiniest hit and be begging for mercy while heart is pounding out of chest. It can go either way. It doesn't depend on the strain, that's irrelevant. It depends on the individuals personal physical reaction to THC, among other things like their history of abusing/using the drug and for how long. Mainly personal neurochemistry.
The anxiety symptoms I always got from weed were physical in nature - my mind would race a bit but the root of the issue would be that I was uncomfortable with the physiological effects that weed had on my body. Not just the increase in heart rate, weed generally made me feel like complete shit. I believe that it actually messed directly or indirectly with the part of my brain that is responsible for 'fight or flight' response, the amygdala or whatever...
I still smoked through the anxiety which was stupid, I am a person that does stupid things like that. For whatever reason I still smoked over an ounce a week because when I built up a tolerance then I didn't get anxious anymore. But then the anxiety would surface when I was sober, and I had previously never had sober anxiety before. So there was this conservation of anxiety thing going on... I couldn't escape from the weed anxiety. Either I smoked occasionally and my highs would be more like panic attacks, or I smoked day in day out, had a great time, loved my herb but then I would experience really bad withdrawals. The withdrawals would obviously include anxiety, due to the anxiety that I had always had smoking weed in the beginning. Smoking more weed wasn't just making that problem disappear - the problem that I shouldn't have ever touched weed to begin with since it's such a garbage drug for me with horrible side effects. I shouldn't have been smoking weed in the first place, due to the panic issues it caused me. But, the way it plays on the reward system of the brain is similar to opioids and cocaine and other psychologically addictive drugs. So I was hooked even though I thought it was a stupid drug basically. Which is weird, I got hooked fast too. Quicker and more seriously and intensely than with opioids, which I have no trouble controlling to a reasonable level when I use them for my chronic back pain. I've been sniffing bumps heroin for years due to my chronic pain and never have I experienced anything remotely like the intense cravings, obsessive thoughts, and physical withdrawal symptoms that I had with weed and I do use opioids daily. I wish I knew about them earlier in life... I would have thrown my bong in the garbage with a smile. Plus, I wouldn't have extreme anxiety right now because the opioid high doesn't have anxiety as a side effect.
Now I suffer from chronic extreme anxiety and panic disorder, which I never had before I smoked all that weed. There is an obvious correlation between the psychotic break that I had and how stoned I had been for many years. It surfaced when I quit blazing. No I wasn't self medicating with weed. As I explained, I was getting really high all day and then I had a withdrawal which involved panic attacks. The panic attacks never went away, I have them 24/7 now. The only time I was ever physically anxious in life was when I was stoned. At no other time did I ever feel like I was having a heart attack or stupid shit like that. I feel that way 24/7 now (well I would but I need to be on meds for it). So this weed abuse caught up with me big time and it is my greatest regret that I ever smoked weed in the first place.
So for the people who get anxiety when they are stoned but not when they are sober, I would recommend quitting weed. Having the experience of panicking / extreme anxiety repeatedly while you are high can eventually lead to an anxiety or panic disorder when you are sober. Plus it's just stupid because why would you smoke something that makes you anxious (in my experience it was because of the cravings, it can be really addictive). Just know that it's a very common side effect.