I was just looking at this chart, where it show all the window period of HIV/STD tests: http://smartsexresource.com/sites/default/files/SUPERCHART5.pdf
Your symptoms do not sound STD related and with negative tets see no reason to think that is it.
read the article at the bottom of the page by dr jose gozalez- garcia can you get hiv from doing this may help clear some issues very informative. ive read where syphillis can take up to 6 months to detect sometimes.gono and chlymidia seem to me they would have been detected easier due to the idea they are looking for the bacteria .herpes some say is easier to detect during an outbreak.i dnt think hepc would cause thick semen but you mentioned lower back pain so maybe something to consider.keep in mind green tea,st johns wort and coconut oil are antiviral atibacterial antifungal,and boost the immune system .they may offer you some relief of your symptoms and slow down infection until you are able to get the answers you need. good luck .