Does my breast reduction affect my period?

Yes, surgery is a stress to the body and will have impact on a number of hormones, especially cortisol.  Cortisol is a stress hormone that increases following trauma and injury.  It has knock on effects to LH, FSH and estrogen and may well impact on your menstraul cycle.

Don't worry, it will return to normal once you recover and the stress response resolves.  If not, see your physician who can have a more in-depth consultation and assessment.