Should I take the shot?

I'm 17 years old and I'm thinking of getting the shot but I'm not exactly sure since I've seen so many negative comments about it and so many positives. I weigh about 90.5 pounds and I have excessive periods to where i have horrible cramps on just the first day then I'm good for the rest of the week. I would have to go home from school for the day because the pain spreads to the legs. Would this be good for me? Can I just take the shot once for 3 months and go back to normal?