Hmm- I am going to suggest that "feeling better" after one day on doxy is just because you didn't wait long enough for your body to recover from the cold virus. Colds can last from 7-14 days, depending on the specific virus and our own immune systems. This is very typical of people who keep a "Z-pack" on hand, start taking it on day 5, 6 or 7 of a cold, and believe it cured them. Actually, time cured the cold, and they took unnecessary drugs.

I stopped the antibiotics for MAC at the end of 2019 (I was still MAC positive). At that time, I was sure I was going to have frequent exacerbations because it was my "pattern" for years before my MAC & Pseudo were diagnosed and treated.

I talked to both my ID doc and my primary about having an "emergency" script like I with some other meds for treating my asthma and frequent herpes flares. Both said "NO!" and gave me many reasons, but especially because I did have this history, and had been on many antibiotics, some effective, some not. Their logic was that treatment is available everywhere if I become ill (I travel over 6 months of the year), or by phone consult with one of them. They wanted no chance of over, under, or incorrect treatment, and forming yet another antibiotic resistance.

Last week, I came across an very interesting article about who succeeds in MAC treatment, and it talked specifically about the other infectious organisms present in those patients. My takeaway is that we really need to back off on antibiotic use unless absolutely necessary, because overuse can and does actually kill off beneficial organisms. So, I am pretty sure my docs were right in their approach - you can read the article here:
https://www.ncbi.nlm.nih.gov/books/NBK537076/#
May I ask you what additional steps (above typical airway clearance) you took to push back the exacerbation and let your body recover from the cold virus?

Sue