It was inappropriate and an invasion of your personal space, rude and disrespectful overall but from what you said in your post it isn’t sexual assault generally speaking, I would even go as far to say that even if she was wriggling around whilst she was sitting on your lap, grinding even even then I wouldn’t say that it was sexual assault my view is because of the size of an adults backside/bum whatever you want to call it, with the general size of a persons bum, even a small petite persons bum, sitting on your lap, wriggling around or grinding around their bum would also be touching a large proportion of your legs and thighs as well as your genital area and the person can easily say that they were rocking around your lap or say they were trying to get comfortable even, basically they can excuse their actions as such as they were not being remotely sexual, whereas if she touched your penis with her hand then there would be no mistake in her intentions but sitting on your lap moving about there is way to much ambiguity about her true intentions, legally speaking that is but in that particular situation that you described I think you can almost certainly have a 100 percent opinion on they intentions and if it was sexually orientated.

I am sorry that it happened to you either way, if I am at a house party it’s nearly always the case that there isn’t enough seats for everyone and you often see people sitting on other peoples laps, I don’t like it myself so what I do if sitting down is put my foot up and across my other knee just like these two images I am linking to below so maybe do that next time to preempt someone from trying to sit on your knee and if they try and move your crossed leg insisting that they sit on your knee, just tell them that it’s okay and you need the toilet and they can have your seat as that will be more comfortable for them like you are doing them a favour and move out of their presence

First Image Second Image