Just curious, since you had a lift, if your PS placed permanent sutures inside, kind of like a bra holding your breasts up? If so, ask your Dr if your weight gain could possibly have ripped a suture or two out? Just a thought. Ask if it could be seen on a mammogram if it was done digitally (the best kind to get anyway) or if could be seem by any other method other than surgery? Ct scan, ultra sound, x ray, MRI. Might be able to have it covered by your insurance if they are diagnosing the disorder as opposed to plastic surgery. If not, the only way to find out would be surgery to open you up amd look. At least your last mammogram was clear, that's the best part! Keep us abreast of what your Dr finds!
I would suggest seeing your plastic surgeon as a first step. These symptoms are unuusal at this late date and need to be evaluated.