My 3rd pregnancy, sometimes i've got a baby bump sometimes i don't?

ok, in the morning when i wake up, or after ive been laying down or napping, i roll onto my back, and have a baby poking out! but if i lay down during the day to check, nothing....its not gas or bloating, it is a hard as rock baby bump. i have been pregnant twice, and have never seen this. when you show, you show, right? sometimes, its two lumps. i think im about 14-15 weeks, and i've been seeing this lump in the morning for a few weeks now. its getting bigger....but will still dissapear during the day. sometimes there's two lumps, and one side is bigger than the other. has anyone ever heard of this????? i have had no other feelings or typical signs of twins...but now i'm starting to wonder? the lump is usually....the size of a melon, and i thought the baby would only be a few inches long at this point? what is going on?