Female Bengal Humping Arm

I have a female spayed Bengal that befriended me, She has started to hump my arm quite insistently. At first I thought she was attacking me but it is clear she is mounting me. Either from on her back or from above. Why is she doing this?

She likes you?

It could be that the spay was not totally cleaned out; piece of uterine tissue left to think she's a whole female.

How long ago was she spayed?

Also it could be a dominate thing - not a sexual one.

i'm not sure how long ago she was spayed. she just jumped through my window one day and made herself at home. i don't feed her because she *must* have an owner somewhere close and i wouldn't want to interfere with that. she is usually at my house for around 20 hours a day. she comes and goes as she pleases. she has been my friend for about a year now and i've never seen her on heat (or pregnant) so i figure she has been spayed. maybe it is a dominance thing, nothing else seems to make sense. i've searched everywhere on the web for female cat humping but i haven't found anyone experiencing something similar. i've also spoken to other cat owners and this is not something their females have done. i guess she's just a little gay
thanks.