Odd behaviour

Question
Hi, I have 2 guinea pigs and was assured at the time of buying them that they were both female. This morning however I noticed 1 is acting strange, she is the quite one, although neither particularly like being picked up, I went in to let them into the run in the garden as usual and she was very reluctant, she is usually the first into their box,   Once out I noticed her back end was shaking quite fiercely also she was lying flat on the ground, when she tried to move she would keep her nose on the ground as if to push her back end round and every time she moved her back legs she shook quite hard, I picked her up, easily done cause she didn't run away as she would usual, and wrapped her in a tea towel and just sat and held her, I tried to give her some fresh green leaves but she hardly touched them and each time I moved my arm she looked about as if she was happier with it against her.  I have put her separate from the other one and the rabbits whom they normally share, I have given her lots of clean bedding, water food and greens but I am worried.  Do you have any ideas what might be wrong?

Answer
if she is kept with rabbits then there is a reasonable chance that she has been injured and that her legs or possibly even her spine is damaged, it is quite a common thing with guinea pig, I would make sue she stays hydrated, has food closeby and that you book her in with a local vet soon.

but...

one thing to  make sure of first is that she is a female, otherwise it could also be a rumblestrut, which is what males do when trying to attract females, but you would have probably seen this from the first day of having them.