She does get upset when she is on her belly...yet, that is all she does the entire day. =) She has never liked tummy time. Is it just something she will get used to? On the few occasions that she has fallen asleep on her tummy, she has slept better.
My LO hated being on his tummy and when he started rolling in his crib, he hated then as well. I think babies feel stuck and that upsets them. Once he got used to it, he loved sleeping on his tummy and that's how he sleeps every night. It's a new thing for them but they usually get over it :)