Morning Sickness theory

I am 18 weeks as of yesterday and still getting morning sickness a few days a week. I had it every day from weeks 9-16, but the past few weeks it's been here and there. I have noticed a trend however....on the mornings when I get up and have to wash my hair, I throw up! This week I tried washing my hair at night and my mornings were great. Yesterday I did a morning wash to check, and I threw up! Is this a weird coincidence, or could there be some sense with my theory? This is my first pregnancy.