My 21 month old daughter was always a good sleeper at night. The last month she has been getting up crying. Not sure why?

COULD IT BE. A lot of times changes in the family, a move, an illness, death in the family or allergies can disrupt sleep. If atmosphere is not calm and peaceful , this could affect sleep patterns, also change in caretaker or people came and went are all possibilities. Provide more specific info as to give you more specific answer.
Not sleeping well. Sometimes if there are changes in caretaking, new entry to daycare/ preschool setting, they do this. Sometimes just after falling sick the start not sleeping well. Sometimes teething can also throw off the sleeping schedule. The trick is to keep the sleeping schedule the same, not to pick them up and talk to them in the middle of the night when they are awake.