Baby not sleeping well?

My darling daughter is 6 months old and she usually sleeps soundly, only wakes for one maybe two feeds a night but the last 3 or 4 nights she is waking every 1 to 2 hours and she is quite upset when she does. I have checked she is not wet, not hungy, she is comfortable, not teething and everything is in its routine. Does any one know why this could be happening? How can I help her.