Hi @machelp, I don’t have MAC but I’ve had to be on antibiotics longterm and also, recently developed C-diff while on another antibiotic.
If you have watery stools you should have a sample lab tested for C-diff. That requires another type of antibiotic specific to the C-Diff bacteria. To get a collection kit, call your doctor for a lab order. You may need to stop by the lab to pick up the kit to take home, and then return the next day with the sample.

If C-diff is ruled out, start eating a good quality, live culture Greek yogurt daily such as Chobani. Preferably plain yogurt. Fruity yogurts contain too much sugar. You can add fresh fruit or small amounts of jam, juice, to give it flavor. You control the sugar that way. Even Activia can do in a pinch to help restore normal gut flora. But it is important to have active yogurt cultures.
Also, eating pre and pro-biotics daily can really help ramp up the suffering gut biome depleted by the antibiotics.

Prebiotics allow your gut bacteria to produce nutrients for your colon cells, which leads to a healthier digestive system. Here is a great article with a list of prebiotic foods.
https://www.healthline.com/nutrition/19-best-prebiotic-foods
And a list of healthy pro-biotic foods https://www.verywellhealth.com/probiotic-foods-7970825

It’s also a good idea when you have diarrhea to avoid milk and dairy products. Yogurt is an exception because the lactose is neutralized in the process of making the yogurt. Kefir is another great product for gut biome. However, if you’re not used to it, the side effects can be a bit troublesome with serious gas and bloating.

I hope you find some improvement soon! Have you been tested for C-diff?