What exactly are braxton hicks?

I'm 19 weeks and I get this tight feeling in my lower abdomen, right under my belly. It tightens up so much that it's like a lump and it feels like one of the babies is lying there but they aren't. Each time they only last like 20 seconds but they are uncomfortable and they only show up when I stretch or orgasm. Does this sound like braxton hicks? I'm not exactly 100% sure what they are.