What Causes Headaches After Applying Hair Dye?

I dyed my hair 17 days ago and started experiencing headaches 2 days later. I've been having headaches for 15 days in a row and have no idea what to do. It is the first time I dyed my hair one solid color (I've gotten highlights in the past though and tried to dye my hair at home, although I never experienced headaches afterwards). The headaches were worse in the beginning, they were so powerful I got migraines and began to throw up, but they decreased in pain since the dye is fading. The problem is that I know the headace is there and even if I don't have a headache I fear that I will eventually get another one. But I've been washing my hair constantly to try and get the dye to fade quicker (using lots of shampoo and warm water). Thank you.