M.L.
You can let her cry it out, sneak in to rub her on the back for a few minutes, but not pick her up or anything. Though I wonder if maybe she has an upset tummy, perhaps a milk allergy or something? It often is more upsetting at night b/c they are laying down for long periods of time.
Perhaps you can get better advice here:
http://www.askdrsears.com/html/7/T070100.asp
Also, around this age, children start to wake more frequently b/c they are more aware, getting frustrated with trying to walk and such. It's actually a milestone for night waking to reemerge at this time in their little lives.