Looks like buck grease to me. My males get it on a regular basis. I bath them with a soft cloth, kitten shampoo and a soft toothbrush (mostly for the tail) to help remove it. I read somewhere that a small amount of dish liquid can help remove a real bad case of the grease.
I've never heard of it on females but if it's protein related then perhaps so. But bathing gets rid of it on my males and I bathe them as often as required for them to be clean. Sometimes there's not much of a buildup and it takes a long time before there is a problem with the buildup - it just depends. Her back looks bad so I'd suggest bathing her now
Mygirl has buck grease when she is in a cage with other females. She is extremely territorial and has to be housed alone (she tried to kill Meeha no exaggeration had her pinned and was biting her throat) Since I have seperated her she is happy and healthy and no more buck grease.
In my experience, buck grease can be hard to get off even during bathing. But with a little patience, I found that massaging the back and using a combination of the toothbrush and soft cloth to help loosen it, the grease will come off. In bad cases I've had to do a second washing back to back to get it all off. Just be persistent but gentle - it'll come.
I feed my rats a Suebee's-like grain mix, and they definitely prefer it over dog food. So I don't give them too much of that. The dog food is always available, and they get some grain mix every day, not enough to fill up on for the whole day, but enough to keep them healthy (at least, I hope!)