I just wanted to say that I had the same problem with mine. It got a little better when she learned to roll back onto her back, but it didn't go away until she learned to get up on all fours. Basically she is learning some physical movement and once she achieves the feet of rolling over, she feels stuck. What I did was try to find something else for her to do when she would keep rolling over, like put her in a swing, play with her, or even better, start helping her learn to get up on all fours (or just roll back over)...then they have hours of fun rocking back and forth, and it is really cute.
I also noticed that just before she learns something new, like getting up on all fours, crawling and walking, she will go through a couple weeks of grumpiness. I think it is just frustration. Babies know what they want to do before they are physically able too.
When she is crying in her tummy, try to interact with her, help her build some back and neck muscles (sore muscles are another reason for teh grampyness, think about when you start working a muscle group that you havent been using lately) Give her some massage.
Hope that helps a little.