Why Do Pimples Appear On Labia?

I am a 62 year old female. For the past 10 yrs or so I have had a pimple appear in a couple of places on my labia - one at a time - occasionally in the year. One at a time. It is red and uncomforable for a day then slightly tender for a day. By the 3rd, 4th or 5th day at most it has gone. It seems to have a tiny white/yellowish centre then by the second day and then gone. No scab.

Can you tell me what this is. A nurse once looked and said it was just a pimple or slight abrasion as the skin gets more fragile as one gets older.