You could have nicked a capillary or something, probably not a big deal but be careful it doesn't keep happening, you don't want to be getting any estradiol directly into your bloodstream. According to my doctor the chances of blood clotting skyrockets. I do sub-q injections in my stomach and one time I noticed some bruising starting after the injection, it was no big deal and I just sort of took note of what I did differently that time. Haven't had any issues since.