Food allergies/sensitivities.

One female acquaintance lost a significant amount of weight because she felt ill from eating. She was found to have a corn allergy. Corn allergy is difficult to accommodate because corn (and corn syrup) is nearly ubiquitous in processed foods and even used as fillers in many medications.

Gluten is a common food sensitivity, although there are others, Sometimes a person has multiple food allergies/sensitivities which can make it harder to identify.

Keep a food journal, or get tested.

By the way, with all the antibiotics you are getting, here is another consideration- antibiotics and the gut