Is my female cat humping me?

Ok she's done this to me and some of my friends before, but I really don't think she is showing dominance because she is the most submissive cat I've ever seen. But when I'm sleeping sometimes she goes up and down my leg and at the same time kneading the bed like she would a stuffed animal. When she does this she purrs REALLY loud, it really grosses me out. She isn't spayed but I read a question on a spayed cat doing this too. Why does she do this, and IS it humping?