I think that might be dopamine rather than serotonin but similar results. It's an addictive reward chemical.


Dopamine is the "anticipation" neurotransmitter.


You felt a lot of things. During acute stress, so many hormonal changes are happening, that your body is completely overwhelmed by them and it takes a long time and a lot of other hormones to neutralize the previous states and eventually bring you back to homeostasis. If you feel some sort of euphoric state, it might be rather due to endorphins (endogenous morphine), which is usually secreted during prolonged heavy exercise, acute pain, sex, and other stuff that people refer to as "it's better than sex".


This is totally normal, anxiety is all about fear in anticipation, your brain is telling you everything is scary and dangerous, you had an experience which demonstrated that you could do it and be successful contrary to what your brain was saying, so the anxiety immediately dissipated. It's like pulling back the curtain on a magic trick. Hold onto that feeling! When you're next feeling anxious you can use it as a reason to push past the anxiety. You might not have a success every time, but it still probably won't be as bad as the anxiety was telling you it would be.