Allergic Reaction Out of Nowhere! Be careful!

I was on compounded sema for about 6 weeks and stopped for two weeks and switched to compounded Tirz with the B12. After my first tiny micro dose shot, I immediately had a terrible allergic reaction! I have never had a reaction to medication before. Be aware that this is something that can happen! I had no idea it was even a possibility, and really wish I had considered that first before paying for 3 months of the stuff :(