I have a similar question. It has happened to me twice now, again about 15mins ago when i had my shot. I inj in my glutes, aspirate when i stick the needle in and proceed when there is no blood. Once i have finished injecting i aspirate again, and there was a bit of red blood when i did. I find it difficult holding the needle straight, but for the most part i keep it steady. Where is the blood from? It can't be that i've hit an artery, because there's no real pain.