A friend of mine (1st ex-wife) nearly died from an infection caused by a sponge being left inside her 2 years ago following routine surgery. Ended up losing a portion of her colon and the law suit is still in progress. Seems an MRI is in order to check you out inside, think a sponge would be invisible to an x-ray.
I wouldn't let the same surgeon open me back up, I would seek a second opinion so as not to give them the opportunity to destroy evidence.