if one thing is certain with babies, it is uncertainty. the minute they get in a routine, something happens and it gets switched up.
many reasons she may suddenly be waking; the first and probably most important is that she is probably really hungry. so just feed her and get back to bed, or feed her in bed if you are nursing. :D that way you are both relaxed and peaceful. :D it really really works.
babies tummies are tiny. they have to eat often, especially if they are having a growth spurt. so have patience.
another thing is every developmental milestone interrupts sleep. not sure why. rolling over, sitting up, walking, all those big ones that babies go through. even teething can start at 3 months, even if you see no teeth for months. my son started having the pain around 3 months, but no teeth till 7 months.
so just have patience. this will also change. you know, the whole "this too shall pass"