Every time i wash my hair i get a headache. why is that? is it the shampoo or something else?

Would depend: on the headache. Frontal headache with sinus fullness.... a sinus congestion/irritation. Often people move their head and neck a log w. washing hair and strains of the neck can give headaches inthe back of the head. Tension typically in the forehead (haven't found that to be true) New headaches are a reason to seek evaluation Without an actual examination, I cannot give specific medical advice.

Possibly shampoo: You listed Migraine headaches in your history. Different triggers you might be experiencing can be temperature, smells, chemical exposure, stimulation of the occipital nerves and/or the trigeminal ganglion. Change your shampoo first, see if that helps. If not try altering the temp of the water. Lastly, if these are not effective see your PCM and consider a referral to a headache specialist.