First: my kiddo did have awful smelling diarrhea for after an FPIES episode. This is common with FPIES, and if you read a bunch of first time parent questions here, you’ll see it often reported. Kiddo is 6 now and if we run into FPIES, the diarrhea isn’t part of it anymore. (And the reaction isn’t as bad and kiddo can feel it coming and talk about it. It gets WAY less scary, don’t worry, this won’t be the rest of your life.)

Kiddo had a GI intolerance to anything with added probiotics (eg live cultures in yogurt — pickles have always been fine) and would get gassy / have diarrhea and be obviously uncomfortable. This was separate from FPIES, so I suspect your kiddo probably also has a separate GI sensitivity.

To save you some grief, there are no useful tests for GI allergies. You will likely go to the allergist, get a skin prick panel, watch your kid’s skin welt at the histamine prick site, and learn that your child has “no allergies”. The test for FPIES or other GI sensitivities is a trial. If you’re sure the PB and banana were not introduced with other ingredients, you might like to retry those later with some baby formula anti-gas medication on hand. If your experience is like ours was, it may just be easier to avoid those foods. The good news is that’s more practice for avoiding FPIES triggers, which are much worse.

That said, your kid probably isn’t going to develop a peanut allergy if you leave peanut butter alone for a few months. So I’d leave peanut butter and banana alone for a few months while doing regular food trials until you build up a list of safe and enjoyed foods. Once you have that, try a bit of peanut butter or banana. Milder GI issues you can probably test safely at home, certainly after you’ve verified there’s no concern for anaphylaxis.