You really need to see a melanoma specialist for those lines on your nails. Because that is one of the more common signs of melanoma.
As for HIV, it doesn't sound like you had a risk. And you had 2 negative tests. I really think your first concern should be getting checked for melanoma.
A condom slip usually is not a risk if the head of the penis was covered. I don't know how long after the event you took the oraquick tests but if used correctly at the right time, you aren't going to get two false negatives.
Hiv is not diagnosed by symptoms. Lines under your nails can be from different things. Sure, melanoma I guess. But it is more likely do to splinter hemorrhages or breaks in skin. But get that checked.
The only ways adults get HIV is from unprotected vaginal or anal sex or sharing of IV drug needles. Keep that in mind. And if someone has a risk of unprotected sex one time, the odds are still very low that they would get HIV at less than 2 percent.
You were never at risk for HIV transmission. The fact that you know when these lines appeared on your nail after your event only means that you are very closely monitoring your body, which offcourse is not a usual behavior.
Your problem is leaning towards cyberchondria - You had sex, you got anxious, you imagined irrational situations, you read through a lot on the internet, you started monitoring your body and you are now obsessed.
Condoms are sufficient to protect against HIV transmission, semen can't magically crawl up from the condom to inside of your vagina. You didn't need to test.
Please see a therapist, it might be a good option to work this out with a professional in order to get rid of the fear and anxiety.