About 3-4 Months Ago, I Got 1 Itchy Spot On

About 3-4 months ago, I got 1 itchy spot on the side of my leg at the beginning of my period. Each month on the first day of my cycle the spot comes back. It is slightly raised, itchy, red. During the next few days of my period, it fills with clear liquid like a blister. It goes away on its own and comes back each month same spot.
I don't use tampons. I haven't changed the pads that I use. I am not on daily medications.