I have had this really mild achy/pressure a little when i pee on the outside of my vagina and just when i am out and about what could this be?

Take A Good Look: It might be a good idea to get a small mirror and take a look in order to be able to see any rash or blisters or irritation, etc-- By what you said it could be one of several possibilities, most or all of which can be taken care of. So look for redness, blisters, any bulging or any lumps you can feel. Also any discharge or spotting. Hey! You'll then have become an honorary Gynecologist!