rat growth?

Hi, I was wondering if anyone could help me. I just came home from a three day vacation and noticed that one of my rats has a round ball like growth of somesort behind her ear. Initially I thought it might be some sort of tick or something, but it looks kind of like a mass of dried blood. I tried to take a picture but none of them came out. It doesn;t seem to be bothering her or causing her pain. I was wondering if someone had an idea of what it could be or could direct me to a good source to do some further research.

Try washing it off - if she scratched her ear for some reason, it could be just dried blood?