If you forgot it, it can't have been for anything very significant. You wouldn't have forgotten a doctor visit for brain cancer or something that really mattered. If it was for a cold or a sore knee, forget it. Maybe mention it to the AME who will yawn and move on. The FAA are not going to search your medical records and snatch your license for this.
Or, of course, you can pay mega $$$ to someone like Dr Bruce who will make a big deal of it and 'rectify' it with the FAA or you could listen to Ron Levy who will tell you to hire an attorney to put it right.
I am not advocating deliberate falsehood, I leave that to Dr Bruce (see the treadmill thread on the red board), I am just saying it is probably no big deal.
Stephen.