What Could Severe Headache With Sore Neck Suggest?

I have had a really bad headache for the last 3 days. It started as just a headache and now I am worried. It has progressed to the back of my head and my neck feels a little sore. It doesn't really bother me until I bend over to pick anything up.. Then it just pounds. I started a diet a few days ago and thought it was a hungry headache but I have stopped. Last night I woke up and It was really bad. It would be sharp pains when my nose would try and clear as I was turning over in bed. What could this be?