For 2 Days I Have Had An Unbearable Headache Only

for 2 days I have had an unbearable headache only in the back, left side of my head w pain halfway down left side of neck pain has been building and head and neck are tender to the touch. headache is a stabbing pain opposed to a throbbing pain. bright lights and sharp noise or fast movement of head results in a momentary, blinding pain.
I have tried otc pain meds, stretching, cold and hot compresses, hot shower - no relief. I am really starting to feel afraid because ive never had anything like this. I dont know why- but I keep thinking its a blood clot - probably because that would be the worst thing.