Some of my cats----who are all "fixed"--- get together in a sensual/sexual way. It is somewhat related to dominance but also involves elements of play and just sensual enjoyment. Altering a cat changes the hormones, but they are still present to some degree and the behavior is part of their hardwiring which can be activated by social or hormonal or seasonal cues.

I never worried about it much as the cats settled matters among each other without fighting.

Sometimes a fixed cat will go through a false heat... your vet can help you with that, but she may have just gotten stirred up over something and reacted with cat-passion.