I have a problem with my favorite cat please help!!! i dont know how long i have?

I have a sweet domesticatedred (DSH) cat and she is getting up there in age. she is 12 and she has a slight health problem. She is obese and the vets have done everything they can to help she gains a little weight now and then but nothings substantial. Thats not her problem, for 12 years she has gone to the bathroom in her littetrbox and now for some reason she is making (number 2 ) all around the litter box but not in it?! i dont know why. and she is acting weird about everything. she follows me around and she always is meowing and i dont know why she is behaving diffrent . Please help my mom said that if we cant find whats wrong we have to put her down. All answeres are appreciated and please be serious im in some trouble. :(