Many reasons... anything from allergies to heat to heart problems.
Unfortunately we can't give you an answer on here other than to consult your vet if this behaviour continues.

Please take her to the vet! It could be serious, and she may have a serious illness or simply may be mildly dehydrated, but only the vet can help you and diagnose it. Does she have any other symptoms, such as her eating and drinking habits- have they changed? Has she urinated or had a bowel movement? Is it too hot where you live? Please consult your vet!