A genital herpes outbreak can show as a rash with no blisters. BUT the very first symptomatic outbreak is usually the worst with painful blisters, fever, swollen lymph nodes. I also googled std rashes, and according to the cdc website, second stage syphillis can manifest as a rash.
As said, the best thing to do is go get tested. Save yourselves the traumatic worrying and get a FULL std screening. Make sure they know to test for hsv-1 and hsv-2. They are not automatically tested for.
Ronjon3, because you shave every other day and if the lube was a new one, I think you are having an extremely bad reaction. Your genitals are extremely sensitive. Of course I agree you should get tested just in case (specifically hsv because of the pus filled bumps) but I also shave. I get irritate easily if I use dull blades, the wrong creams, and also shaving too often.
No std presents "rashes" persay. Genital Herpes presents lesions and Gential Warts presents Warts. If they had puss in them it is a possibility that it could be herpes since they starts off as vesicles then turn into blisters which contain puss which then ulcerate, which then dry up. Unless your bf was unfaithful I wouldnt jump to any conclusions though. Go to the doctor have him/her look at the rash. None of us can physically diagnose you better than a doctor. I know it can be very scary and a little embarrassing but if it is possibly an std you wouldnt want to let it linger.
This could be a simple rash due to irritation but it could also be an STD your best bet is to get tested... your BF should as well.