My abdomen is tight???

Koru00

I'm 18w5d and have been noticing more and more (along with a friend who is very excited & had to touch my belly last week) that my abdomen is much tighter/solid/harder than my stomach area.  I've heard and seen women carry low, middle, and high before, but I didn't expect for that to happen (if that is the case) till later on in the pregnancy.  I don't think I'm carrying the baby high because every time the Dr listens for the heartbeat he always hears it in my stomach area. I'm really confused.  I have no pain in that area at all it's just tight/solid/hard.  Any ideas???

 ps I am a little overweight so I haven't been showing that much.. my mom told me my entire stomach abdomen area was rounding alittle if that makes sense.