Can you put a one piece outfit on her, or put a onesie under her clothes so she can't get at her tummy? I think the baby lotion idea is a good one, maybe keep a look out for one for sensitive skin. As for the cause -- I'm not sure, but could she be rubbing against something (the rug, furniture, etc) while she moves? I once got awful rub burn when I was little from sliding down the stairs!
Oh, and I would most definitely check with your ped just to be on the safe side if it doesn't appear to be getting better. If nothing else he/she might be able to prescribe something that would make it better.