My poor yorkipoo is balding on one side. What could be causing this?

[Irie Pirate]

She's 5 years old, spayed, a little overweight, and has allergies.

She had fleas at one point but that's taken care of.

Please help, I'm worried about her and I plan on making an appointment first thing Monday. Until then, I'd like some ideas as to what could be causing this, and if other yorkipoo owners have experienced the same thing.

Here's a pic of her: http://twitter.com/Irie_Pirate/status/27...10/photo/1
She is eating, drinking, and eliminating perfectly fine, and I've always been feeding her the same brand of food, and this just now happened.

For the extremely rude answerer, I for one, did NOT purchase the dog, it was a gift from a family friend. And I don't care what breed she is, she is my baby and I love her dearly and I'm concerned for her health. She has a loving family. Take your pitbull and SWERVE.