I disagree with your friend. Having a lack of feelings associated with a vague memory does not make you a psychopath. I am not certain why your friend believes this but I don’t think it is accurate.
It is difficult to explain memory. Scientists know only a small fraction of how the brain operates. No one can be fully certain of what a memory is. Why did you have that particular memory at this point in time? One possible theory is that you had the memory because you were psychologically ready to have it. Some psychological theories posit that painful memories are unconsciously blocked as a way to protect the conscious mind from having to deal with them. In other words, the memory may have been blocked by your unconscious mind because it was too painful to handle. Perhaps you were psychologically able to handle the memory and that is why it occurred. That is only one theory and it may not explain what happened. There are many other theories to explain your experience.
Some individuals have similar experiences except their memories come in the form of dreams. After a series of dreams, each one more revealing, an individual may believe that he or she had been sexually abused. I mention this with caution. I can’t be sure that your memory is accurate. It is important to keep in mind that memory can be faulty and unreliable. The fact is you may never be able to verify whether the memory has any basis in reality.
The other aspect of your question is why your memory had no emotional affect on you. That too is difficult to answer. I do not believe that your non-reaction means anything negative about you. You should take this incident at face value which is you had a vague memory and you have no emotional connection to it. I do not think you can ascribe a deeper meaning to the memory.
If more memories surface, you may want to keep track of them. It may be a one-time event and as I remarked above, you may never learn the truth. Should more memories surface, you may want to consult a therapist to further explore why they would be entering your conscious mind at this time. Thank you for your question.