Sounds like an epic douche, and you sound like a really trusting person.

Don't feel too bad - I've trusted people with way more than that. Like loaning money to a friend over and over, small amounts and somewhat larger amounts. Truthfully I knew he'd never pay me back even though he said he would each time and asked in manipulative ways. I just felt sorry for him because he is a super dysfunctional human.

My rule is never to lend money I can't afford to lose. That way no major harm done. Sounds like you just lost a bit of gas money - hope it isn't too big of a deal for your bank account.

Step away from Dr Google and don't panic. It could be anything - they'll figure it out.

You could have IBD as others have mentioned (crohn's, colitis) - could be sooooo many different things. Just chill and get it looked after.

I'm by nature an extremely anxious person. CBT helped - catastrophizing is not your friend.

Good luck and hope it turns out to be something simple.