Many photos were taken of this flower, Canada Toadflax [Nuttallanthus canadensis or Linaria canadensis], with trouble getting clear shots due the small size of the petals. It was found behind the plant on 16 May 2013, and other earlier dates in the month. Newcomb’s Wildflower Guide, pages 50-51, says it can be found in sandy soil, and it was found on a hill in that type of soil. It’s a member of the Figwort (Scrophulariaceae) family.
USDA Plants Listing: https://plants.usda.gov/home/plantProfile?symbol=NUCA