What Causes Muscle Weakness, Fatigue, Stiff Neck, Fainting And Rapid Heartbeat?

I have muscle weakness and fatigue, i have a stiff neck, and it feels like my neck is swollen on the inside because it is hard to swallow. I also have been feeling like i am going to pass out when i exert myself even in a moderate intensity(i.e. light exercise). i have also been getting an occasional rapid heartbeat which may be related to the caffiene that i drink in order to make it through the day lately.