PID isn’t always caused by an STI. It’s the most common cause, but PID can occur without ever having had an STI. I can’t explain why your gynaecologist told you that STIs are the only cause of PID because it just isn’t true.
If multiple STI tests were negative but you were still symptomatic then we would refer to gynae as we don’t have the means to swab the upper reproductive system. If you were diagnosed in clinic with PID then the antibiotics you received would cover chlamydia as well.
I would also hazard a guess that the private test might have been a false positive. False positive/negative results can occur on any test, and if you had multiple negatives and one positive then I would be inclined to take the positive as a false result. Chlamydia very rarely self-clears at any site.
However, the latest evidence base suggests that vulvovaginal swabs are more accurate in detecting chlamydia than endocervical swabs.
Sorry you went through that 