I noticed my CFS increased exponentially after an stomach infection

On November I was able to walk many hours, all day shopping, and then resting the day after with no significant PEM.

After a stomach infection (fever and diarrhea) on December, I started to feel severely fatigued, and just able to walk 10 minutes and then my legs started to feel weak and tremble, with hard PEM next day. Now I am bed bound.

Anybody with the same experience? How could I revert it if it might be caused by a stomach infection? I also used antibiotics Nefuroxazide.