Thanks for the help!
It wasn't a lot of blood and I'm pretty sure it was the rat's. Ok, I'll make him wait for the next week for two weeks. Just to make sure that he stays healthy.
Thank you for the information.
I researching other places to buy feeders from. I'm sure there are some.
Well, maybe one of my roommates might have left the freezer open and didn't tell me? I don't have another explanation

The weight loss is probably not that bad. The first time I weighed him was three days after a big meal (he hadn't pooped in a while as well) and then the second time was after he had a big poop. I timed it very badly. That's probably the reason for the weight loss.
Thanks for the tipp.
I'll definitely will not use water as hot as that anymore.
To the feeding source: I have a "Fressnapf" near my home. They have different supplier brands and feeder sizes which gives me lots of room to choose from. Personally I really love the brand called
Petmad but it's hard to get the feeders from them since they're not always available.
If it's okay, I'd still like to take you up upon that offer with your friend in Germany. My friends don't really like or care about snakes which is why no one around me has an idea where to buy from. I'd love to hear from someone else where they get their feeders from.