reruns back to pelvis in 1 and half months post delivery and never returns back to prepregnancy size as there is hyperplasia
of uterine muscles,tummy returning back to original size depends othe amount of weight gain I pregnancy and the postpartum exercises you do,tenderness might be nonspecific or there might be collection or other infection some where else in the abdomen ,as these symptoms are four months after you were operated,so leas likely that it could be related to cessarean but the possibilitys cannot bbruled out would advice you to get a sonography
done and others blood test
and urine test to rule out other causes of fever
and raised ESR.