I've always had an interest in menstruation and this was made even more intense watching a friend of my sister. She often came to stay with us and slept in my sister's bedroom which was next to mine.
One morning my sister was in the bathroom and I knew her friend would be dressing. She had left the door open a fraction, just enough for me to peek at her without her seeing me. To my amazement and great excitement I saw her drying herself then watched as she attached a looped sanitary towel to an elastic belt. She stepped into it and adusted it, making sure it was snug between her legs.
My cock had already sprung into action so I stroked myself through my clothes as I watched her putting her bra on then climbing into a pair of strange plastic lined panties which I later discovered were sani-knicks. The idea of her wearing plastic pants over her pad took me over the edge and I spunked into my underpants.
I soon started 'borrowing' pads from my sisters supply to wank into and her friend was always the subject of my fantasies. This fetish has stayed with me to this day.