Burning sensation during iv heroin/meth use (Not missing or hitting an artery)

[RottingWithin]

What's up guy's and girls,
So, I've been having an issue the past few months that I find a bit odd. First off, some info on me. I'm a 31 year old male and I've been an iv heroin and meth (always mixed together) user for about 12 years. The H. is in tar form around here. I use new and clean rigs every time now but in the past I had a terrible habit of using one rig for a month. As you can imagine, I did some damage to my veins but as I've always used excellent form (meaning almost always hit the first time even with damaged rigs and rotated sites) they're still usable for the most part.
Now, onto my issue...
This only happens when I hit on my lower arms, from the elbow down. I hit the vein, register and start to depress the plunger. It's all good for about half the shot then all the sudden I get this intense burning sensation that is always a few inches from where I'm hitting (toward my heart). I have a high pain tollerance and this isn't enough to keep me from finishing the shot but it is definitely enough to make me stop for a second and curse my veins. Obviously I'm not missing as the pain is in a different location and I'm not in an artery. I've hit an artery before and this is a completely different feeling. This happens every time I hit my lower arms, no matter which vein I use causing differing levels of pain. The most painful one is when I hit my right forearm and the pain is in the crook of my elbow where I started my iv career. And I used the fuck outta this spot, trust me. This leads me to believe that the issue stems from my past stupidity. Could the vein be leaking?? That to me sounds unlikely for various reasons but that's why I'm posting here. Maybe someone more knowlegable on the subject can educate me.
Thanks to anyone who took the time to read this. I encourage all to respond with any info/ideas you've got or just to tell me I'm a moron. I've got thick skin, I can take it. Just be creative with your insults, please.

 

[tacodude]

Possibly scar tissue causing restriction to the flow putting pressure on the vein as extra fluid tries to pass through. Just a hypothesis and in no way an educated guess.

 

[flowersonfire]

Is there any possibility that you may have went through the vein and some is leaking through? I have done that - burns about half inch above because when I first went in, I apparently punctured through to the other side. I register and about half way through I feel that terrible burn and have to stop and quickly find a new vein. That's a total pain in the ass!
(If anyone has any tips on avoiding that I'd be thankful to hear. My veins are thin and hard to hit as it is. I like the rush, and don't get it if I use a leg vein, so I try to use my arms)