Sounds like typical Braxton Hicks Contractions to me. If they stop when you lay down, that is a good thing. If they were to continue, and become even more painfull, and regular, then you should start to worry. And if you have any blood, then worry. Or if you start to lose your plug, worry. If you start to lose, it, it will be almost like a snotty texture. Like when your sick and you cough up alot of flem. I'm not trying to be gross about it, but that is really the only way to describe it.
But remember, the further into your pregnancy you get, the stronger Braxton Hicks seem to get.