As Nicola says get a piece of white tissue and dab her bits, if she's in season you'll get some staining on the tissue, can be from very dark red to pinky watery discharge. Her bits will be swollen too, but if its her first time and shes just started you may be confused as to how swollen is swollen ifswim (not something you'll be taking too much notice of). I always notice spots on the floor too and on their bedding. may also be "crusty" round her back end where she's not cleaned properly.
Failing that, if she has been clippered its not impossible to have nicked that area too if you can see they have been clippering round it.
