What could cause rapid weight gain if I recently increased my physical activity? I am also having issues with heartburn and mild fatigue. Related?

Heartburn or angina? There may be a relationship. The increased activity may be straining your heart---heart burn can be mistaken for heart pain called angina. The weight gain may be fluid retention because of strain on your heart, which also causes fatigue. Or it could be several unrelated conditions. You need to call your doctor for a check up ---and do it soon.