You can try putting her to bed earlier to see if she sleeps a little later. Try between 6 and 6:30 or so. She should be getting between 11 and 12 hours at night, if she's not, that might be the cause of early waking.
Or, if you're putting her to bed at 7 or 7:30, you could continue to do that and see if she returns to her normal waking schedule on her own.
If she's taking feeds at night, you could also play with these a little bit too. Another thought...does she usually have early growth spurts? If so, this could also be the cause. At 6 months there's usually another big one.
Hope this helps
*HUGS*
Sonya =P