My 5 month old chiuahuah is bleeding thick red blood from her rectum and she won't eat or drink anything what could it be?

It can be time to rush her to the VET. Too many possible causes, many life threatening.

My guess would be hookworms or parvo. The only way to know for sure, is to talk to your vet and have your vet look at her. An exam is not expensive.

It means that your puppy needs to go to the vet asap.