You're more in danger of getting colon cancer from modern refined-food diets, inactivity or obesity than you are from seven days of taking way, way too many antibiotics. How did you even get them, did you go to several different doctors? (If this was all from your regular doctor, it's amazing they would let your anxiety run the show. Your anxiety is more of a health issue than the risk of getting cancer from one week of antibiotics.)
To support your gut biome, take probiotics for a month (go to the health-food store and get the kind that is kept in a fridge). Also, just for your general ease, know the symptoms of colorectal cancer and ask your doctor to put regular fecal-occult testing in your normal checkups.
I'd also suggest, see a counselor and talk over what's going on such that the cheating happened. A lot of times, people stressed or anxious about one thing that's harder to solve or nail down, transfer their anxiety to something medical that's easy to pin down, even if it's not the real cause of the anxiety. It's possible that having cheated, if it's not OK in your relationship, led you down the uncomfortable emotional path of feeling like a less than honorable person. It's lots easier to freak out about STDs or colorectal cancer than to ask yourself why you did something like this when in a relationship. So, talk to a counselor about what you want in a relationship if cheating and being dishonest with your partner happened. If you address the discontinuity between what you're trying for in a relationship (being an honorable man) and what you actually did, your fears about STDs and CRC will likely be seen for what they are, a smokescreen, and go to the proper place. That place is, always stay slightly alert to all medical possibilities, but don't let fake concerns run the show.