Possible exposure

Hi  I had oral sex and received cum in my mouth but did not swallow .  About 2 to 3 weeks from that time I noticed a rash on the lower middle of my back , the rash consisted of small red dots about 1 to 2 mm in dia. The dots disippeared from my lower back and some showed up on my sides,  about 30 or so.  I have not had a fever or any other signs but after the rash showed up my under arms got uncomfterable for about 20 hrs. This all started about 3.5 weeks ago and the rash is still visible but has improved. I got a 20 min HIV test 2 days ago and it was neg.  I also had std test and all that was neg. What could cause the rash and do you think it is ars. My test was about 5 weeks from pos exposure and I am going to get another test at 7 weeks.    The rash has me very worried.

[Teak]

http://www.medhelp.org/posts/HIV-Prevention/Possible-HIV-infection-Symptoms/show/1198198?personal_page_id=1287266&post_id=post_5480828