Whats wrong with my cat?

she used to be a lovely cat, would love snuggles and always want attention. but over the last 3 months she has put on tons of weight, when i try and pick her up she squeaks in pain and doesnt want to be held anymore, shes growling alot at people (she never used to do that) and last night she normally lets me know she wants to go out to the loo but she pee'd randomly on the floor. shes 3 years old and has been spayed. im taking her to the vets tomorrow but does anyone know what this might be? :(