I Had A 20-30mg A Day Opiate Addiction/dependency For A

I had a 20-30mg a day opiate addiction/dependency for a few years. I only took enough to feel manageable. I knew I needed to stop so I seeked help with a addiction specialist/doctor. He put me on Gabapentine, chlonodine, and another medication used for seizures, but had off label uses for anxiety. So I quit and was feeling great, I went to all my follow ups and they tested my urine twice there was no trace of opiates. I feel I was pressured in to getting the Vivitrol injection, the night I received my first injection I thought I was going to die. My heart was pounding, my body was hot and tingly. I had such stomach pain it felt like labor. My question is, I know it last for one month but does it weaken each day? I really want this out of my system, now I feel stuck. I'm a 47 year old female that weighs 120 lbs. do they inject the same size syringe in to everyone? I know it's pre-filled but do they have different mls? Any feedback would be helpful