The bumps are about nickel to quarter sized and when you touch them they're fairly tender. She doesn't bite or anything, but she flinches a little and tries to move her neck away from me. They have gotten a bit worse but nothing major to the point of calling the vet (I call him in emergency only if at all possible).
They don't seem to affect her being able to be in a frame or round up when we are doing dressage and she has no hesitation grazing.
Do you think this could be an allergic reaction to something? She has been in the same pasture all winter/spring and the only thing I have added to her diet was a product called Accel which is a multi-vitamin.