Why does my cat meow all night long?

Well not all night long but at random times fo the night she'll just meow loud. She is fat. About 18 to 20 pounds. I have food available for her at night? I dont know if shes in pain or what? She is spayed. Im nto for sure if this has to do with her meowing but before she got spayed, she was pregnant. wich for the vets did not knwo and neither did we. Well they did some xrays to check and see fi everythign was alright and she was preggers! She also doesnt like to be held like a baby and usually not at all. Do you think that those babies inside her are making her nto holdable and making her meow? She is about 12 years old? Maybe 11?