K.
I wasn't able to recognize my braxton-hicks until I was around 30 weeks last time. This pregnancy I started feeling them around 18 weeks because I knew what to look for. Yes, it feels like a "tightness" in your lower belly. If you push on your uterus it actually feels hard.
As for when to worry, if you can't really "feel" them, you don't need to worry. If you feel like you're getting a lot then make sure you're well hydrated etc. The concern is if you go into preterm labor. You'll know if this is the case - I promise :)