In rereading your question, I think I misinterpreted it. Apparently you don't have documented UTIs, just recurrent symptoms of painful urination and back pain. Is that right?
The lack of response to antibiotics suggests not that you are infected with resistant bacteria, but that no bacterial infection at all is responsible.
Have you been checked for genital herpes? Are there any open sores or other lesions when you're having symptoms? I doubt herpes is the explanation, but perhaps it should be checked out to be certain.
Welcome to the STD forum. I'll try to help.
As someone with recurrent UTI, you probably have learned that sex often triggers episodes. (Hence the old term "honeymoon cystitis".) Such women are colonized with UTI bacteria like E. coli and others in the vaginal area, and during sex the bacteria can be massaged into the urethra. Most likely that is what is going on now -- nothing related to your partner's recent sexual infidelity.
1) The main symptom of gonorrhea and chlamydia in women is vaginal discharge, not painful or urgent urination. These STDs can mimic UTI, but usually do not. If your symptoms are typical for your past UTIs, that is probably what you have.
2) It is wise for any people who are sexually active outside monogamous relationships, or are the partners of persons who have other partners, to have periodic STD testing. On that basis, you should be tested from time to time. But I doubt it is the problem now.
3) Nitrofurantoin (the generic name) is not usually active against gonorrhea or chlamydia. Still, it might interfere with testing. Probably best to wait until you have been off the drug for a couple of weeks.
4) The obvious explanation usually is the right one. I'm sure doctor would agree with my comments above.
Returning to your recurrent UTI: After repeated treatments, it is common for the bacteria causing UTI to become resistant. Given the delayed response to treatment, most likely you were infected with a strain resistant to nitrofurantoin -- which these days is not the usual drug used for UTIs anyway. You might talk to your doctor about other possible treatments in the future. Second, if your UTIs regularly are triggered by sex, you might take a single dose of antibiotic after intercourse. (I have personal experience with this. For several years my wife had recurrent, sex-triggered UTIs. Taking a single low dose of ciprofloxacin was highly effective in preventing her infections. After a few years, she was able to stop and the problem didn't return.)
I hope this helps. Best wishes-- HHH, MD