I had umbilical hernia surgery on jan 28 and a month later i was laying in bed and coughed and felt a pop. what could have happened?

See your doctor: This could mean nothing but a feeling of pulling on internal sutures, especially if mesh was used at surgery. However, rarely hernias can recur & would need to be examined by your doctor.

Depends: If there was pain at the umbilical hernia site and a bulge, then this may be a recurrence of the hernia. Recommend evaluation by the surgeon who did the repair.