This happened a few years ago.
I was invited to stay with a girl I had met on the Internet, she was just a friend and had a free room at the time. We had chatted quite a bit and sometimes got a little flirty but that was as far as it went. When I arrived I was shown the spare room and we got something to eat, as the evening went on we got very flirty but she told me we couldn't have sex because it was her time of the month.
Anyway come bedtime we decided to sleep in the same bed which was fine by me. As soon as the light went out we started kissing and to my surprise her hand went straight to my cock. I was fully hard in seconds and she started to stroke me. Soon we shifted position so that I was on my back and she was sitting on my legs. She started to really masturbate me which was the first time another person had done so. I reached up and helped her out of her shirt so I could see her large full breasts and watch them sway as her hand moved up and down. I was very close to coming and said as much but she was happy for me to make a mess. I relaxed into it and soon shot my load all over the place.
After that we slept and the next night did the same thing again. We stayed in touch for a while but lost contact.