Your heart is most definitely doing good rhythm wise then, the only thing left to rule out cardiac issues would be an echocardiogram but I wouldn't worry about it unless you want it to completely rid yourself of anxiety related to something being wrong with your heart. You are a young lad and your heart should be just fine
I believe this is not only anxiety but post-MDMA binge problems you are having,
how has your sleep been doing?
The pins and needles in your hands is likely from oxygen problems in your peripheral nervous system, too much oxygen is a big cause of pins and needles that start in the extremity. This is technically known as
respiratory alkalosis. When you breathe too quickly your blood fills up with oxygen and not enough CO2, which means that even though you have lots of oxygen in your blood stream your cells actually can't use it because your cells need CO2 to do the gas exchange basically. So you have so much oxygen that your tissues are being deprived of oxygen.
Have they ever taken your blood oxygen levels during one of these episodes? Its counter-intuitive because you will be gasping for air when the problem is actually too much oxygen. This is common with panic attacks though and is a big reason why breathing quickly makes things worse. This is a big cause of feeling lightheaded with panic attacks, blood flow/oxygen to the brain is very reduced during respiratory alkalosis episodes and blood flow to the front part of your brain is reduced for around 6 weeks after MDMA as well. This ain't a good combo.
You could think about buying an
oximeter (probably 30-40$) to investigate this respiratory alkalosis theory, I'm sure your oxygen saturates when you're having an attack anyways but it would be helpful to have it on hand if you are exercising and also to check your blood oxygen during your dizzy spells. 100% blood oxygen definitely ain't the goal here.
That you could be having problems with pins and needles in your hands associated with anxiety hints that the respiratory alkalosis is likely very real, but adrenaline constricting your blood vessels reduces blood flow to the nerves as well and can kick up pins and needles. This lack of blood flow and oxygen can make for bodily pains all over, serotonin dysregulation aside. When your muscles don't have natural oxygen they find a way to make their own oxygen and this results in a mild burning.
Its good that you're walking and getting out there though, I would continue to take some light walks and stay hydrated with electrolytes, don't eat too much garbage and avoid alcohol. If you can pin down that you're having oxygen problems with the oximeter I suggest exercising more whole heartedly (no pun intended), but I wouldn't be surprised if it makes you light headed with weird body sensations like pins and needles sorta until you get your breathing under control. Any questions fire away, good luck.