The small pokes from the needle are irritating but we typically use among the smallest needles available, so the pain from these should be minor. If the pain was from the poke, you may simply be more sensitive than some other people to this stimulation.
If your pain was from the actual injection of medication, that is unusual. Asclera should be essentially painless as it enters the vein. This contrasts with saline injections, which hurt substantially.