Never heard that overthinking can make your body feel weak. As for voimiting I guess you feel suffocated when you think too much which can lead you to feel vomit.I don't know for sure, just an assumption
Hey it happens to me.... And I have seen this in many people...... I don't know if this is normal or not but you are not alone who is facing this.... there are many......
Stress can upset your digestive system and can make gas.... which makes you to feel vomit.. ( I am sharing my experience what I feel )
I vomit before main exams... I never eat anything before exam. I vomit in stress. And even I faint down.... My body temperature increases and for weeks I am on bed sometimes..... This is all because of stress.