@seven09 Well, as for why your PCR tests continue to be negative, probably because you don't have it this time. If one person had one negative test, I would say possible error, but several negatives among you - probably not.

As for still feeling ill in spite of being tested for 8 different viruses, remember that there are thousands of viruses that affect people, there are only accurate tests for a few dozen of them. So quite probably you have one of the many for which there is no test. My daughter (school nurse) and her 1st grader & 3 year old have all been walking virus pots since school started a month ago, but no Covid - just congestion, wicked coughs, and tired. We were at the school yesterday, and it seemed like half the kids and several adults had coughs, were hoarse...

The biggest question is whether to attend the important birthday party --- a personal decision. In my family, I expect to see this play out over and over again until the indoor season is over. Here are some well-respected criteria we consider:
If you are symptomatic of any illness - fever, rash, cough, vomiting... - stay home from indoor gatherings that will include vulnerable people.
If you have recently been ill, but the symptoms are gone (3-7 days for most viruses) - attend but keep your distance from vulnerable people. Maybe consider wearing a mask and keeping the sanitizer handy.
If you are part of a household or office that has a virus, but you have remained symptom-free - should be safe

Our decisions on gatherings are often made hour-by-hour - unless you have an absolute deadline for deciding, can you wait til the weekend, letting family know why?
Sue