belly keeps getting really hard with a lot of pressure

I can't tell if it's baby pushing against my belly or if it's a contraction because it does get really hard & it's somewhat uncomfortable..Only reason I don't think it's baby pushing against it is because it's hard all over with a slight tightening/pressure sensation for a few seconds(30-60 seconds) ..Reason I think it is baby is because it really only happens when Im laying down lately..I'm almost 31 weeks.