Random flashbacks of abuse?

tw: sexual abuse

I just suddenly got this very random flashback and it is really bothering me as to why I get them. I am 19 now, but this memory was way back in middle school, I think. I was riding a bus that wasn’t normally mine because we had switched for some reason. I was sitting with this boy that I knew and I remember that he was repeatedly trying to touch me. I told him no numerous times and he still persisted. Thankfully his stop came and he was about to get off but he called me an obscene word before leaving. I remember trembling and feeling so scared after he left. I don’t know why but I had completely forgot this even happened.

This isn’t the first time I have had this happen, I have had flashbacks of sexual abuse before but my mind can’t really determine if I made it up or not since the abuser is my sister. She is really nice and has always treated me well so I struggle to come to terms with the idea that she was the person in memory. She is a devote Christian, married, and is just a blessed and great person honestly.

I don’t know if those memories are real or not because there’s a lot of them and they all involve her. I don’t know why my brain will make up something like that. All of the memories are very vivid and hard to believe that they are fabricated.

What does these flashbacks mean? Why are memories coming up suddenly from long ago? I know the bus one was real but I’m not sure about the rest....