Your experience sounds very similar to what happened to me at age 14.
I'm a male human being whom at 14 began to have sexual fantasies of being female. This only happened to me while in an aroused state. Afterward I would have my normal feelings back and find the other state repulsive.
After a while it stopped happening then, later started again then, went away again.
I now believe that this was an alternate personality who was female.
I have chosen to acknowledge her existance in the collective and welcomed her back so, that she can express her feelings to me.
Does your doctor or therapist know of these feelings and when they started?
Do you have a doctor or therapist?