How can I figure out what is disturbing my baby's sleep?

Too many answers!!! Disturbed sleep in babies is incredibly common. This is clearly a question to address with your pediatrician or family practice doctor. It may simply be your child has a low threshold for arousals. Formula/milk reflux is quite common as is formula and milk protein intolerance. Simple things like broken finger and toe nails, constipation and diaper dermatitis abound. Is parenting a baby great!
Ask her! Seriously, babies often signal by non-verbal clues what may be triggering her sleep disturbance. Is she tugging at her ears? Is she drooling excessively? Is she feverish to touch? Are her eyes weeping or tearing unusually? Is there a peculiar new rash? Any of these could signal an ear or throat infection, for example. Sometimes, though, you cannot isolate the cause without the help of your doctor.
Disturbed sleep. A baby' sleep can be disturbed by three general reasons. Either he is dirty, hungry or in pain. As a mother you will soon be able to determine the reasons.
A few thoughts. If she's < 4 mos. Old, you have time to prevent sleep problems by putting her in her own crib in her own room, drowsy but awake, on her back. Self-regulation to fall asleep helps consolidate sleep. Babies cry out during light sleep & have many more sleep cycles/night than adults. If she's a healthy 6-mo.-old or up, night-waking & night-feeding are learned behaviors. Ask your pede if ok to ignore.