Welcome to our Forum. I'll do my best to help.  Let me provide comments both regarding your risk for STIs and your symptoms or redness and discomfort in your groin. 

First your exposure.  Your risk for STI is rather low.  Most masseuses, even those who perform commercial sex acts do not have STIs.  Further, some sex acts are more likely to lead to infection than others and even when a partner has an STI, most single exposures do not lead to infection.  In your case, there is no risk for any STI from receipt of masturbation and virtually no risk from condom protected vaginal intercourse.  STIs are sometimes transmitted through receipt of oral sex.  When oral sex leads to STI, the most common infections are gonorrhea or non-gonococcal urethritis, both of which cause symptoms of urethral discharge or burning on urination (not urinary frequency and not the sort of groin discomfort you describe). 

Second, your symptoms.  No STIs would cause symptoms which begin within a day following exposure and few, is any, cause symptoms at two days.  The frequent urination you noticed is not a sign of STI but is not uncommon with anxiety.  Similarly, the burning sensation that you describe in the fold at the inside top of your legs is not an STI symptom but does sound like a fungal infection ("jock itch").   Also, when STIs cause redness and sores they would not normally do so at the creases at the top of both legs.

My sense is that you have a very strong sense of guilt from the low risk event that you describe.  I would encourage you to see a clinician who can help you sort things out.  I doubt that you have an STI but would encourage you to get tested for the STIs that you might have acquired from receipt of oral sex, i.e. gonorrhea and chlamydia.  Nothing you mention raises concerns for any other STIs and testing should be available for you in Japan. I suspect that you can also access some over the counter antifungal medication which will relieve the discomfort in your groin.

I hope these comments are helpful to you.  EWH

Hi Doctor thank you for your prompt response.

Your comments about likelihood of infection and risk around my exposure has gone some way to put my mind as ease if not completely removed my anxiety...

It’s been six days now, a combination of sudocrem and talc/powder during the day seems to have helped with my potential fungal infection, the itching/burning has subsided with the redness and dry skin lessening as well.

I have found an english website here in Japan who provide home testing kits for both gonorrhea and chlamydia which I plan to undertake.

I have developed a small flesh coloured lump on the shaft of my penis at the base, it’s quite pronounced and a couple of millimetres across. I appreciate this could be nothing as I am obsessing but could this development after 6-7 days be a sign of a  STI of some kind like warts etc?

Thank you once again for your assistance