Thinking of Quitting

My daughter Alexa is now five and a half months old and has decided to only eat for five to ten minutes maximum per feeding. I'm breasting feeding so this can get pretty frustrating. During the day she doesn't eat as much but, the last feeding before I put her down, she wants to eat for half an hour! This is fine but, since she doesn't eat enough throughout the day, I'm finding that I'm not producing enough milk for her. To make matters worse......she refuses to take a bottle. Therefore, I can't even pump and save the milk. Is this common? Is this just a phase that she's going through?
She has always been a great eater and usually eats for a half and hour or so. I'm getting worried that maybe my milk is going bad. I don't want to quit giving her my breast milk because I know it's the best thing for her but, I'm almost getting to that point!
Please help me and give me some sort of explanation for this unusual behavior.

Angie