I had a shocking headache for 5 days at about 5 weeks. Dr said it was hormones. Also last pregnancy I had my first ever migraine (what hell it was, I never knew they were that bad). This was due to the softening of the ligaments in my neck. I am expecting I'll prob get one or two this time around.
Its the same with back pain, everything is softening, so you're more susceptible to pain.