Can i be allergic to the breast shield of my breast pump?

It is possible: Most current shields are made of soft silicone. While usually not allergenic, it is possible to develop allergic contact dermatitis. Older shields were of harder silicone or latex, both of which are more likely to cause local reactions. If you have sensitive skin or are dermatographic (pressure on the skin causing a welt), just the pressure of the shield may cause local welting.