Hard belly

I’m 16 weeks 4 days and sometimes my belly feels hard and when I’m laying down that’s when you can really feel the firmness. But sometimes it doesn’t feel hard even when I’m laying down. I carry most of my weight on my belly so I already had a little belly before pregnancy.

Is it normal for that to happen? Could it be that the baby moved further back in my uterus and that’s why I can’t feel it? Or is it just more inconsistent because baby is still small and it’ll feel more firm as the pregnancy progresses? I’m a FTM so little things make me anxious.