My 4 year old son is potty trained, but recently he has had an accident for the past 3 nights. three days ago he had surgery, what could be wrong?

Normal reaction: This sounds like a totally normal reaction to a stressful situation - surgery. It could have also come from anesthetics given during the surgery, but more likely a little bit of regression/fear from the stress and change of routine surrounding the surgery. I would not make a big deal out of this, be calm and supportive and this will likely go away with time.