Sooo maybe it’s not premenstrual symptoms after all? I’m in a long-term relationship and (as far as I know) monogamy isn’t an issue. Any ideas what’s going on?
First off, this may or may not be related to a sexually transmitted infection (STI.) Bodies and menstrual cycles are diverse, it’s best to see your doctor or an STI testing clinic to be sure.
You can get tested for STIs whether or not you are currently experiencing the symptoms you described. That way you can either rule out STIs (including chlamydia, gonorrhea, or trichomoniasis) or treat any present infections even bacterial vaginosis or yeast infection if necessary.
You may also want to think about testing as a couple. Did you both go for STI testing before you started a sexual relationship? When was the last time that you both got tested? Once you and your partner know your statuses and complete any prescribed treatment, you can stop worrying about unknown STIs.
As you eluded to though, not all long term relationships are mutually monogamous. If one partner has sex with another person outside the relationship, it’s possible to bring STIs back to the relationship. That’s why we also encourage people in long term relationships to make routine testing and safer sex tools like internal or external condoms, sex dams, lube, and conversation a part of their sexual habits.
Hope this helps for now, but really you’re next step will be seeing a medical professional to rule out or diagnose what is going on for you.