It upsets your gut because it kills the beneficial bacteria in your gut which is needed for digestion of nutrients, inflammation reduction, fermenting carbohydrates, strengthening your intestinal wall, stimulating your immune cells, regulating hormones, etc. Doctors fail to tell you this when they prescribe the typical antibiotic and decongestant for a sinus infection. BTW, your own body will produce a healthy antibiotic anyway without the medical antibiotic, and in 7 to 10 days (length on prescription) you'll be better anyway. The medical antibiotic is sold as a convenience, but there's a natural convenience too that doctors used before "medical" doctors...
If you choose to take their antibiotic (not recommended), I'd follow it up with a Probiotic of 10-20 (10 different strains and 20 billion CFUs) to replace your gut bacteria. I'd also eat yogurt, cabbage, or anything high in probiotics.
Instead of using the antibiotic in the first place, I'd suggest researching and understanding what colloidal silver is. Realize most sources will misinform you that it will make your skin "turn blue". This is utter nonsense and a lie. Millions of people take colloidal silver, millions of people are not blue (like the famous "blue man" they show on the fake news all the time). Argyria, cause of "blue skin", is caused by contact or ingestion of silver chloride, not colloidal silver. You can buy colloidal silver at the grocery store or online and avoid your doctor entirely for your sinus infection (among many other things).
They don't like that and will usually lie to you to scare you into getting their prescriptions. I have firsthand experience with this. I had regular sinus infections in the past, before being educated. I tried colloidal silver the first time, but then went to the doctor hours later for the congestion symptoms. He said I no longer had an infection, then got visibly angry when I told him I took colloidal silver earlier that day. He proceeded to tell me that my skin would turn blue, and actually told me that he had "a whole family" in months prior that were all blue from taking colloidal silver (research the number of cases and causes of Argyria in the United States and you will quickly see that that is an absolute BS lie). He suggested that even though I didn't have an infection any longer that I should still get the prescription for an antibiotic. I had sinus infections 1 to 2 times a year in the past. I haven't had a sinus infection in 10 years. I take colloidal silver whenever I need to. I'm not blue.
If you take it when the symptoms first start you won't get a sinus infection (hold under the tongue for 30 seconds for better absorption into the blood stream). If you already have a sinus infection you should be able to knock it out in a couple of hours easily. You will still need to deal with the congestion, but only if you wait too long before killing the infection. You won't need a prescription for the congestion either. Look into turmeric and honey remedies for the congestion.
I'd recommend "Sovereign Silver" (I take just when needed) at the grocery store or online. Also get a 10-20 probiotic for daily use. Reduce the amount of GMO foods you eat.
Colloidal silver has many uses other than sinus infections (kills viruses, kills "bad" bacterial infections, kills fungal infections, wound healing, water purification, pet medical care, kill many diseases, etc.). It is not dangerous. It is put into babies eyes when they're born to prevent infection. It isn't called colloidal silver though. They just make up some other name for it. Other products use the term "Silver" in their marketing, not telling you it actually contains some amount of colloidal silver (like some band-aid brands for one). If you like keeping your money then you can just get a bottle of colloidal silver, which will last a good while.