Legs and knees shaking


So every time my bf and I are doing things ( fingers me or goes down on me ) I seem to be fine for the first 3/4 minutes and I'll be enjoying it ..then I'll reach a certain point where my legs and knees will start shaking and closing and I can't take no more! And we have to stop .. i always feel bad for having to stop as well.

Is there a way of controlling it?!

Does it happen to anyone else ? And why does this happen ?