Hard lump in lower right abdo when laying flat?

First time mom here, history of miscarriages so you can say I'm a bit of a worrywort as this is our first that's almost made it to 2nd trimester. Anywho, has anyone else experienced this?

I was laying flat on my back reading a book when I felt a weird (hard to explain) alien movement on the right side of my lower abdomen then visibly watched a lump about the size of a ping pong ball harden and slightly stick out of my tummy. It didn't hurt and was more just pressure and foreign feeling. When I stood up, it went away and my tummy was normal again but when I laid back down I could feel the lump again. Is this normal? Baby shoving itself up or my uterus? Or should I be concerned and make an appointment with the OB?