why even tell her?..its unlikely shell find out..noones going to black mail you as they did it too.
i think you need to calm down, you dont have to tell her EVERYTHING.. id be suprised if she hasnt a similar sort of experience as we all went through a weird phases when we were trying to understand/deal with the changes in our bodies...
, i may be rare in this but i dont share every little detail about my past with a bf, why do they need to know?..as a gf id also feel unsure as what im meant to do/say to such a confession, id probably rather not know as it seems so insignificant
ps of COURSE shes kept stuff from you..dont flatter yourself