Aspirate man. The procedure is there for a reason. Is like when you hold a gun, you never keep the finger on the trigger.
I doubt you put the whole 2.5 in the blood. 2.5 would have make you at least pass out. It was probably a bigger vein and you were very close to it. When you push the oil in, it creates a pressurized bubble. Any nearby open vein will receive a little bit from the pressure.