Sounds like normal stretching. Kinda scarey but don't worry!
That sounds like what I have every now and then (I'm 19weeks with twins). It's just the normal muscle's stretching by the sounds of it making room for the baby. It's find, you will find you will have it some days and not other days
I feel like I have some pressure every now and then and my stomach hardens up....if i touch it, I can actually feel the baby pressing up against my stomach....its very uncomfortable, but thats what it could be!