S.M.
Try Tylenol - she may be in pain from teeth or an ear infection. For ear infections, my kids never showed any other symptom than the middle of the night crying. I always thought it was behavioral at first and then felt really guillty.
Unfortunately, you are also at the age where a lot of babies adjust their sleep behavior. It may jus tbe a matter of putting her to sleep earlier or later. Kids this age also start being anxious, and if she wakes up and you aren't there, she may be very upset. I would do your best to reassure her without picking her up everytime she cries. Try cracking hte door so she can see out a little. It shall pass, just stick to routine and consistancy. And yes, she may be getting ready ot lose the two naps. My 17 month old is almost ready for one I think (darn!).
But look for sources of pain too. SOmetimes it is the lying down tha tbrings on pressure in the head/sinuses/eustaian tubes (sp?).