Pericarditis
 

Has anybody had pericarditis? How long did it take to recover and how hard is it to get your class 1 medical back?

Yes

You may be back to normal in a week but fatigue for months is not unusual

I had mine before I started flying and as a medical student had the knowledge to ensure my doctors got off viral studies which effectively prove what virus has caused the illness. They didn't blink handing out a class 1

You may well find the regulator is cautious about giving you back your license as you can get fluid around the heart and so echocariograms are useful