Your test at 20 weeks was conclusive and there is no need to doubt them whatsoever. I will tell you a fact, even if you were immunodeficient, which offcourse is absolutely unlikely, you would have tested positive at that juncture.
If your partner is exhibiting certain symptoms, please get her to see a GP, it is not HIV unless, you have acquired this virus to pass on to her from an another event that you are not telling us or she acquired it from unprotected vaginal or anal intercourse or shared needles to inject drugs intravenously with another HIV positive person.
Most importantly, symptoms are never used to diagnose HIV infection because some ARS symptoms resemble plenty of other microbial infection, including common cold.
My best guess is, you are overreacting, every thing is okay. I have 2 advice to give. For you, start working on acceptance; you are HIV negative. For her, see a GP.
Soberman, you can not get HIV from someone who doesn't have it. JUST having unprotected sex isn't a risk for getting HIV. You have to have unprotected sex with someone that is HIV+. The person you had sex with is your girlfriend. Not an unknown person. The risk of having unprotect with someone who is HIV - is ZERO. I would have to look back at what your risk was 6 months ago that has you worried.
HIV does not appear out of nowhere. And you were never exposed yourself. You can't infect your girlfriend if you were never infected yourself.
If you’re in the US, you can buy a PCR test yourself. But a 20 weeek rapid sure sounds convincing to me