If she's a belly sleeper, I'd say let her sleep on her belly. Especially if that's the only way she'll sleep for very long. Is she to the point where she can roll over on her own? If not, you can always get an angelcare monitor so that you don't have to worry about SIDs. Check too, to see if she's teething or constipated as both of those can interrupt sleep patterns. If she's going through a growth spurt, she could also be keeping you up simply because she's hungry and not getting enough to eat during the course of the day she she's trying to make up for it at night.
Good luck! And keep in mind that the sleepless nights won't last forever. Eventually, she will grow out of this. ;3
Hi! Was she teething? My 5 month old has her two bottom teeth already!