My parakeet is acting very strange!?!? please help!!?

Okay I have a female parakeet named Belle and she is about two years old. She was very skinny when we got her, but we managed to plump her up a bit. However, today when I went to check on her she was like keeeled over! She feels very light and weak and just flops her body around. She started eating a little, but she had to stop and rest every few seconds. She is not dehydrated because she has water sources all around. What do you think is wrong with her? She looks more skinny than normal, too. She lives with another male parakeet named Ave so do u think she could be pregnat or something?? I don't think so though cus shes really skinny