Is my rat being bullied by her sister?

Question
Hi,

I have 2 sister rats who are about 6 months old.  They have always had little play-fights where they make squeaking noises but no-one's been hurt yet.  

Recently, Fussy (the smaller of the two) has started displaying more dominant behaviour towards Fidget - humping and neck nibbling, but Fidget is starting to protest in a more distressed manner and Fussy won't leave her alone.  Fussy chases her around the cage and continues to hump her until she decides she's had enough herself.  I noticed Fidget was sitting in the corner of the cage today looking very forlorn all hunched up with her ears back staring into the distance and now whenever Fussy comes near her she squeaks or moves away.  She also spends a lot of time trying to get my attention - when it happens when they're both out of the cage Fidget climbs up me to escape.

They seem to still be sleeping curled up together in their hammock, but I'm starting to get worried because Fidget looks so tense all the time and Fussy won't give her much peace and often won't let her eat without coming over to interfere.  I read somewhere that if there's no blood you shouldn't separate them?

Any advice would be grately appreciated.

Thanks,

Answer
This is the perfect time that they start to argue a bit about pecking order.  ITs more common to occur with males but some females go through this too.

However, if the other one is that stressed this isnt good either. It wouldnt hurt to let the live apart but play together daily and even explore each others cages, but keep them apart for a bit till their hormones right themselves.