Sleeping on back. 2nd trimester

Elyse
Okay, so I love sleeping on my back. I'm 15w and 2 days. I have like 0 belly. Just bloated most of the time but it's not hard and when I wake up in the mornings my belly is flat again. I don't know why I don't have a belly (am I okay?) anyway. Since there is no belly can I lay on my back? 
Ps: I just came from an appointment and the baby is growing fine btw. Just little to no belly I guess.