20 weeks & uncomfortable 'stretching' pains when lying on back - normal?

[Didntthinkiwasthisdisorganised]

I'm 20 weeks and have a nice and quite obvious bump that's getting bigger reasonably quickly and in which I've felt some vague movement - very exciting. I'm not sleeping on my front any more so doing lots of sleeping on my back so I can feel for movements, however when I woke up this morning I was aware of a few little aches happening for a few seconds at a time in my stomach, kind of behind the bump, not in one place but over time they happened all over my stomach area. Nothing bad, just uncomfortable, and logic tells me that it's the weight of the bump stretching my stomach muscles out, but right now I would really like to know that this is normal! Now I've got up and put on a new loose top they've gone.
Anyone else had/have this? I know some ladies aren't even showing at this stage...
So is it time to stop sleeping on my back too? for the whole of the next 20 weeks? Is that normal?
And if so, I'm going to need some help... does anyone have any recommendations for particular shapes or makes or types of pregnancy pillows? Have seen a few different types around...

Did it feel a bit tight and hard when it happened? I get this (now 24 weeks) and I think it's early Braxton Hicks (nothing to worry about) where the uterus tightens for a short while- for me because my bump is quite small too, it's especially obvious when I'm lying down. I find sleeping on my side rather than my back a bit more comfortable. I don't have a special pillow, but tend to tuck a warm hot water bottle under one or other hip.