Welcome to the forum. Thanks for your question. I'll try to help ease your fears.
First response, after the opening lines, specifically "So there was no penetration." That means that almost certainly there was no STD risk. They call them STDs because sex is necessary to transmit them -- and that means real sex, not the sort of things you did with your commercial partner.
Mouth-breast contact, massage, fingering, and other hand-genital contact do no transmit these infections: no risk for any and all STDs.
I cannot say the cause of the apparet rash you describe. I doubt it is connected with the exposure you have described -- if it is, the only possibility is an allergic reaction of some sort, for example to a sexual lubricant if one was used. No STD causes this sort of thing.
"Please provide your honest advice." That is the only kind of advice ever given on this forum. And in this case, that advice is to ignore this event, do not get tested for any infection, and continue normal relations with your wife. That's what I would do if I were in your situation.
Do not confuse your obvious guilt and shame over a sexual choice you regret with STD risk from that choice: they are not the same. Deal with the former as you need to, but forget the latter.
I hope this has helped ease your mind. Best wishes-- HHH, MD
Thank you very much indeed Doctor. This helps calm my nerves and breath easy.