My atiel had watery poo :S?

My female pet atiel (I've had her for 2 years), Rosie, has had watery poo lately. In her cage, there's her poo and it's in a puddle of light green liquid. Does she have diarrhea? Should I give her something to help? Or is this just normal? She's about 4 years old. She's a very healthy bird, according to my vet.