Relax bro. and move on from this. Your combo test at 51 days is conclusive.
I have several tests during post exposure 28 days till final at 95 days post. (Around 10+ times of testing due to extreme anxiety.).
The result was not change if no new exposure.
Yes, your test is reliable and accurate at 51 days, You can now confidently move on
You have tested conclusively negative for HIV. Whatever is causing your muscle issues isn't related to HIV, since you are negative. As suggested by the previous respondent, it is most likely anxiety and stress.
You possibly are going through anxiety and stress, which causes muscle tension and pain.