Cry and shake sex

I'm fine when I'm on top
When my boyfriend is on top it's also fine until he really stars grinding  and I don't get any clit stimulation and I only orgasam by clit stimulation but that's besides the point anyway it feels great and my legs start to shake and it starts from my groin area and then I get treary.... I'm not upset or sad or angry 
It just I dunno gets too much? Like in a good way? I can't control it and this is the 2nd time it's happened....
Any clue to why? Or if it's happened to you?