What is this?

I've been having what I can only describe as 'movement' in my lower stomach. 
It started off with small popping feelings that I thought were gas, but it's progressed to what feels like a golf ball rolling around in my lower stomach!
It doesn't hurt, and only happens when I'm sitting or laying down.
I did suspect I was pregnant months ago, but after a negative test and having withdrawal bleeds I kind of forgot about it, until now. 
Has anybody else had this, and it turned out to be nothing? 
Help?