When I was looking into doing a cycle last year ( didnt end up doing it though ) the buddy I was with was experienced. He was explaining on how to do shots, and all the stuff. One thing he did mention was that if you hit a vein and it hits your bloodpath, it will give you a few weeks of a flu like symptom. The suggestion was to inject and drawback, if you see red move the needle, if you dont - then you are good.
WARNING - I am NO PRO - so please - seek other advice as well, but, this info came from a very experienced guy, and I feel you may have done what I mentioned above. Could be a side effect possibly?
I hope this helps!