19 weeks and only feel uterus when standing

So I’m 19 weeks and I can only really feel my uterus (mainly on lower sides of belly button) when standing up. When I am lying down, my tummy looks bigger (it doesn’t go flat anymore) but it’s also a little squishy and most days I cannot feel the top of my uterus. I had my monthly appointment two days ago and the doctor was able to measure my fundal height (it was like an inch below belly button), but since then I can’t feel it. I literally only feel pregnant standing. All other times I feel like it’s a flabby beer belly. I was 135 prior to pregnancy and now I’m 145. Ive been in fairly decent shape my whole life (minus college years!), to give you an idea. Can anyone shed light on this?!