I keep having these false flashbacks and memories of being abused as a child mainly by my father. I even had a dream about it and I "daydream" about it quite often. I wasn't abused when I was a child though, the only thing that happened is that my parents argued a lot. I don't know why I have these flashbacks and it's very bad because they affect the way I see my father even though he would never do such a thing and he cares about me. I feel really ashamed that I could turn such a nice person into a monster inside my own head, I don't know why this is happening
I'm not a psychologist and I don't know you or your life history so this will only be speculation.
There is nothing to be ashamed about. These are psychological events which are involuntary and which have a definite cause and origin for which you are not responsible.
It could be a case of displacement, where feelings connected with one person or thing are subconsciously shifted onto another (in this case, your father).
Perhaps you had sexual fantasies about someone as a child, but repressed them due to their unacceptable nature, and now they are manifesting themselves in this way because you never properly dealt with them.
There is also the electra complex in psychoanalysis, where supposedly the daughter sees herself in competition with the mother for the affections of the father. This usually resolves itself as the girl grows up, but occasionally those early feelings become repressed and are never fully dealt with. This can lead to neurosis and the types of distressing thoughts you describe.
It could always be the case (I'm not saying it is) that someone did abuse you as a child at some point, but your mind repressed it/wiped it from your consciousness to protect you, and so you have completely forgotten about it. Now those memories are finding their way into your consciousness but for some reason have been displaced onto your father. I would think this is an unlikely explanation though.
Sorry if this sounds like half-digested psychoanalysis (which maybe it is).