Itchy sensation could be produced by a std or by many other things. Could be an allergic reaction to any piece of clothes, any chemical product you are applying on the area or the beginning of a yeast infection (but that would be visible).
By the way, regarding your HIV test, you did it 6 weeks after the exposure, if it was a 3rd generation test, it wouldn't be accurate, and you should repeat it 3 months after the encounter. If it was a 4rd generation test, the result is accurate.
Regarding your syphilis test, most people show a positive result between 3 and 6 weeks, but it can take until 3 months, so it would be interesting to repeat it 3 months after the encounter.
I am not saying that the itchy sensation is produced by them, but I would like you to have your results as accurate as possible.
All the best.