My female dog started humping me

I adopted a lab beagle last year and she is 3 y/o now. She has been spayed already, today she started humping me suddenly. She was licking my arm aggressively and started humping me, any idea what could be happening?

We foster cats and the cat we have right now doesn't get along with my dog, we try to not keep them in the same room alone. The cat hisses aggressively at her and my dog is super scared of her. The cat is adopted and she ll leave in a day or two.