How do I keep my baby's diaper from leaking pee?

We cannot figure out why this is happening with our 5-month-old daughter all of a sudden. Often when she wakes up at night, her sleeper and blanket are wet around the small of her back. It is definitely urine and not sweat, so she isn't getting too hot. The diaper is not too small or too big; we are putting it on correctly. She is only in it for 4-5 hours, so she isn't in it for too long. The diaper should be absorbent enough. The only thing I can think of is that she is very very active and moves around a lot in her sleep, but even then it doesn't seem like this should happen.