It is contractions. There is something specific in the mens semen that cause them. Which is why they recommended sex to help bring on labor. Only when it's time will it bring on labor so don't worry! When I get them, they've lasted as long as 20 minutes at times, but they go away.
Sounds like Braxton hicks. It can do that. If it last longer than 2 minuets more frequent than 6 times in an hour go to doctor.