90658 Flu Code

Hello- We are trying to bill for flu shots given to patients in the office, at the same time as their well visits. Do we need a modifier on 90658? I had read that the code 90658 for the vaccine itself, did not need a modifier, but Medicare has denied the first batch of claims I put through. I also billed Q2037 for the Medicare patients- for the actual administration of the flu vaccine.
Thanks