I had and have the same problem. FTM too and my bf is slightly larger....he jus goes slower than we used to and doesn't go all the way. Try start I no off slow and once you feel comfortable to do a bit more than do it. But listen to your body, I can't be on top anymore cuz it hurts. Try different things until you find what works. Good luck...8)
That would be Braxton hicks. Sex can cause false labor pains. Happened all through my first pregnancy whenever I was on top or just very active in general. It's safe as long as you pay attention to how fast they are coming and how hard. Try laying on your back and doing a little less moving..