I think I pulled a muscle in my neck, it got better but I was washing my hair and it popped and now it hurts worse than before. What should I do?

Please see below. The so called "puling of a muscle" is actually usually related to irritation of muscles, ligaments and soft tissue surrounding the spine. As long as you haven't developed actual weakness or numbness of your upper or lower extremities, you wait a few weeks. If you're not better then you can see a spine surgeon (neurosurgeon or orthopedic spine) & get an mri. Plain xrays are often not helpful at all.
Go to doctor. You may have developed a ruptured disc in your neck. Go see an orthopedic surgeon who specializes in necks or a chiropractor. Try to insist on a x-ray to help with the diagnosis.