I Am A 20 Year Old Male And Have Recently

I am a 20 year old male and have recently begun to have a pulsing headache in the back left part of my head close to my neck, but mostly in the occipital region. I have not bumped it on anything that I know of. This pain has lasted about a week and is always constant. Along with this pain I have had stomach aches and fatigue, as well as many hot flashes when there should be none. I am concerned as I've never felt anything like this before and was wondering if it could be serious? What do you think it could be?