I started feeling mine around 16 or so weeks. It's different for everyone. Just make sure you keep hydrated, as that can cause them to happen more often. Honestly there's nothing wrong with having a couple BH every now and then and they are perfectly normal, and from what I've been told they're a good thing and they help get everything prepared for the real deal. Obviously if they are happening in a pattern and more frequent and get more intense and painful you should ring L&D.
Sometimes something as simple as changing positions in the way you're sitting can help them stop. Or going for a walk. I take a warm bath if I have them more often than I'd like.
As for the feeling, for me my belly just gets hard and I can feel a tight squeeze for about up to 30 seconds and then it's soft again. They can get uncomfortable, but shouldn't be too painful where you can't talk or breathe through them.