What’s causing this knot?

Hello ladies I’m 16 weeks 4 days and I have been feeling baby move for about 2 weeks now (light flutters). But sometimes I get this achy cramp that’s kind of uncomfortable but not painful and when I feel my belly it’s a hard knot! What is this? Is the baby balling up? How come is doesn’t feel like the regular movements I’ve been feeling? Help! Lol.