My 8 month old is crying, wailing, whining at night IN her sleep. HELP!?

She's been peacefully sleeping through the night practically since day1 but for the last 4 weeks or so, she's been crying almost hysterically like something is wrong throughout the night and she's been moving in her sleep more than usual. It almost seems as though she's in some kind of pain. The odd thing is that she's crying in her sleep. Her eyes are shut and she doesn't stir when you come in to check on her. I'm worried about the crying and am not getting much sleep myself. The last few nights I've actually awoken her and rocked her back to sleep. Her teeth don't seem to be bothering her at all and she has plenty to eat. Any idea as to why my baby girl seems so unsettled??