What’s that!!???

So I was laying down last night I’m 13 weeks and 3 days along I was lying flat on my back and I noticed this small oval lump in my abdomen slightly to the left I pushed on it and it was harder than the rest of my stomach. Could this be the baby I was feeling? Or maybe something else? I felt the spot again today and it’s not there anymore. This is my first pregnancy so I have no idea.

UPDATE****

Recently had an ob appointment and he said my uterine lining is too thick to feel the baby and I should get checked out if it happens again but I’m 14+ 6 weeks along! The baby is a pretty decent size. Should I be worried??