Feel Nauseated with pain in stomach.

Every day I feel nauseated with extreme acidity. No. of bowel per day is 4-5. This includes partial watery motion and normal consistency. As soon as I eat I feel pain in stomch. After eating a sufficient amount I develop a tendency to poop it out. I have lost 4 kgs weight within a week without any rigorous exercise. I suffer from fatigue. My knees tend to ache to an extent that climbing up or down the stairs become very painful. Muscles have become loose and bones are pretty evident. I have had Giardia infection around 15 years back. Since last 5 years, I am suffering from IBS. I have consulted several doctors and been on medication all these years but nothing really helped. I was tested covid positive 2 months ago.