Maybe a doctor will have a reason. Otherwise, children do often go through unexplained unsettled periods. My DS's are right now. DS1 is waking about 2 times a night (at 4 1/2) for over 3 weeks now; and DS2's waking 3-ish times a night (at 24 months) and waking at 4.20am on the dot every morning. This has been happening for a week or two. I can't put my finger on why. But my children have all been terrible sleepers and I just kind of accept bad nights/weeks now. I have done the doctor thing so many times with no outcome. I usually put it down to teething, bad dreams or big growth spurts.
I know it is really hard. I really do. I have done (and still do) the fitful, sleepless nights. It is soul destroying. But it does get better. There s a light at the end of the tunnel. It will take time though. All I can advise, is for you to sleep with her through the day to catch up and to go to bed early so you get enough hours sleep, even if they're broken.