Old rat fighting with new rat?

Question

I adopted a rat and just bought another rat so she'd have a companion. We've gone through the quarantine and everything and introduced them in the bathtub a few times and everything was great. We put them in the cage together for good this morning and the old one was fighting with the new one a little but they seemed ok. Nothing different than what we'd read. We went out tonight and they were sleeping together when we left but we came home and they were on different sides of the cage and the younger one has a cute on her shoulder blade. Its bloody and the older one has blood on her from it. I've separated them again but now I'm worried. What should I do? I feel awful!

Answer 1

was one of them in the cage before?

if yes they may find that the new one is invading her home