Confused and a little concerned

Toni
Ok so, I've been talking to this guy for about 3 months but I've known him for 3 years... And yesterday he asked me to come over and spend the night.. So I said "ya sure, no problem" so we were talking and laughing.. Popped in a movie and and started holding hands, which lead to cuddling which led to making out... 
Now this is the part I need help with.. 
While we were making out I kept getting this feeling in my stomach that wasn't good.. So I stopped and said I needed to use the bathroom (don't get my wrong, he's an amazing kisser and cuddler).. I got in the bathroom, looked in the mirror and ran to the toilet and threw up... (Wasn't because of anything he did)
Now this isn't the first time it happened.. Both times this has happened I was on my period.. 
So my question is.. Why is it happening?? Is it nerves? Is it hormones? Both?? I have no idea why it happens... It's embarrassing 😫😳