This seems out of the ordinary to me. I've had mine since I can remember talking, and the blocking came later; about third or fourth grade. Oddly enough, my stutter completely left me for about six months between freshman and Sophomore years, but it returned and I still have it now as a 17 (soon to be 18) year old. If it just came to you now I would suggest seeing a doctor as that could possibly be a sign of some sort of brain trauma or neurological issue. (source: my dad is a pediatrician)