How my ex GF made herself popular with my friends [Group]

I am no good writer nor is my english especially good, I will therefore try to make it short.

During last year of school I started hanging out with this girl from my free athletics group I was doing beside the school. She had been there for a longer time but because it was a huge group I never really talked to her before. We had a great time together and on the third occasion when we met eachother privately we slept together. This continued and we started seeing eachother almost daily to “hang out” which basically meant that we spent more time hooking up than talking, but we both enjoyed it alot. I didn’t have much experience with girls before so it was all very new to me and boy did I enjoy it. We both probably thougt at this point that we were partners and took eachother quite seriously.