weird first time on the Depo shot???

I'm 17. Just started on the Depo shot almost 3 months ago now. I've love it it works great for me and I feel great. I need to get a new shot in about a week and I've just got my period for the first time on it, not just spotting but a full on period. Wondering if that's normal?? I have hardly had any spotting the entire rest of the 3 months and it came without warning