Injection pain comes from using improper injection technique first and foremost. If your injection technique is sterile then it comes from contaminated gear. Thirdly it may come from the quality of the oil solvent. Thick oils are more likely to produce pain.
In your case it may be due to the sheer volume you're injecting. However, I regularly inject 2.5cc (due to low conc oils) and don't experience pip at all.
I know I'm probably teaching you to suck eggs here, but the needle tip becomes desterilised the moment it touches anything. Even the bung of the vial you're drawing up from. I'm assuming you use two needles per shot, right? One to draw up and one to inject.
Do you clean the skin in a clockwise motion? You need to swab the skin from the point you intend to inject, and sweep outwards. It's a small point, but the temptation is to scrub the skin, but this technique may displace bacteria from a dirty area of the skin to the recently cleaned area (the point of injection).
Finally, if you're using alcohol swabs you should wait for the skin to dry, because the alcohol kills bacteria by dehydrating their cellular structure through evaporating moisture from the surface of the skin, rather than being directly lethal to the bacteria.
If your injection technique is sound, then consider switching brands. Ampoules are better than vials, as they are single use and shouldn't be contaminated unless the gear manufacturing process is non-sterile.