Dropping pounds when having sex?

I am on my third sexual relationship and I am starting to notice a pattern. Every time I get involved with someone in a sexual way I lose close to 20 pounds. I am five 9 and usually weight about 140/135 but the three times I have been 120/115 were during and after a relationship. I know it's gross when I am that little cause my ribs poke out and everyone says so but I don't know why it keeps happening. Maybe stress?