My guess is behavioral. Since it just 1 ram acting this way, and you don't see other signs of illness, it could indeed be bullying. By the way, in me experience, female rams are quite feisty. If this is the odd fish out, it is possible she/he is taking shelter from the others, regardless of gender.
It could be illness, but without any other symptoms, there is no way to know for sure. Do you happen to have another tank to put her in? Does she clamp her fins or seem stressed? You do have to be careful with this sort of situation in case she is overly stressed since rams are sensitive and stress kills.