Weird Siting

I did a thigh shot today (I had aspirated and all was cool), and left the needle in for a few seconds before pulling it out as I normally do. After about 10 seconds I see some blood in the tunnell of the syringe. I immediately pulled out and cleaned up. I didn't really feel weird or anything and have obviously continued on with my day. The spot does feel a bit itchy, but, of course, that could be the placebo affect. Any ideas as to what may have happened? Thanks in advance to all responses.

Nicked a vein on the way in, no biggie, this will happen several times to ya, esp shooting quads.

In the past, I have seen blood in the tip and once in the tunnel after pulling out. This was weird because there was no blood in there after finishing the shot. All of a sudden the blood shot in there after about 10 seconds of the needle being idle in the spot.