I need help

Of course I cannot go into detail much as I do not want to trigger anybody and I'd prefer it myself anyways... so I am 16 years old and I have a lot of memories (though a good portion of them i always doubt happened, idk maybe out of fear) of my father doing and saying some.. some things to suggest some sort of sexual abuse/assault (i dont know the proper terms) happened. If all my memories are correct and true, that means he pretty much stopped when i was sometime between 7and 9 years old at least that I remember. But now, a month ago on my birthday (June 7th) my dad came over and we, just him and me, went to the store to get some. I was walking just slightly ahead and to the left of him, and he accidentally ran into me causing his hand and lower arm to.. well, touch my butt.. I thought okay well that was accidental but then he did it again... and again... and again... like 6 plus times. It just started to feel less and less like it wasnt meant in a bad way. At one point I looked over to him thinking maybe he was trying to get my attention but he was just looking straight ahead, and maybe it was just me but it seemed he had a slight amile on his face but I don' know...

So with that recently happening, if my feelings are true that it was not accidental, then that leads me to a lot of questions. He no longer lives with me, my parents are divorced and he has his own girlfriend now. The two of them do not live in the same place but she has two kids, a six year old daughter and I think 4 year old son. And if my dad thought it was ok to do that.. if he is the same as he was back then... I am worried he could end up trying the same things with her someday. I'm really hoping my memories are false and my feelings are false... I thought maybe I could report him, don't even know really if that is a possibility or not but, im scared to... honestly the only reason I am consitering it is that girl. My dad hardly ever has just me and him alone, so I am not really threatened besides maybe innapropriate 'accidental' touching. But.. as much as I do not want to report out of fear (i am scared to death of him finding out and then him coming after me) I will feel so much worse if I find out somrthing happened to someone else and I could have prevented it... so guys I need opinions.

My mother knows nothing. I sort of tried telling her when i was 4 but she didnt believe me and took my dads story, and she has told me multiple times "Ik your father is not nice but I dont think he could ever stoop that low" so i dont know if she would believe me if I tried... Ihave a friend whom knows something possibly happened a few times as a kid but she doesnt know he recently did something questionable... i could tell her, I know she will believe me, I'm just nervous that she will end upreporting it to police.. so what should I do?