My baby keeps wakening up fidgeting. Why is this?  

Might not be awake. Your baby might not actually be awake- babies move a lot (and make lots of noise!) during sleep. Sometimes it may sound and seem like they are awake, but they are in fact asleep. If you hear movement or sounds from baby's bedroom don't rush in to pick them up, wait awhile and see if they settle themselves or go back into a deep sleep.
Fidgeting baby. There are a great number of possibilities to explain this. Babies can be irritated by a number of factors e.g. Hunger, changes in diet, sensitivity to foods, changes in temperature, discomfort from teething or digestion problems. Early symptoms of illness may start as just irritability. Irritation of the skin or sensitivity to soaps. If the problem continues consult your physician.