Regarding HIV, they are telling you to go back at 12 weeks, because 12 weeks or 3 months is what most state health officials
consider to be the testing window for HIV
. However, most people usually sero-convert 2 to 4 weeks after contracting the virus. If you were tested at 8 weeks and came back negative, I wouldn't be too worried about it (but I would still abstain from sexual activity, just to be safe, but that is just what I would do personally).