she might be suffering nightmares....both of mine have suffered from them... my almost 4 yr old son still has "night terrors". he basically screams and squeals and cries for 5-30 minutes... it is virtually impossible to "get through" to him. he eventually settles himself back to sleep.
he started this when he was about 18 months.
my daughter is 22 months, almost 23m, and sometimes wakes often at night. usually she only wakes once, sometimes twice. there have been nights, though, that she has woken 3-4 times! she often wakes crying and needs a cuddle... she then settles herself back to sleep. i think she has woken from a bad dream and just wants reassurance that we are still there....
good luck!