What Causes Persistent Headache After A Fever And Neck Pain, Despite The Fever Decreasing And Neck Pain Subsiding?

22 year old man. 3 days ago I woke up with a very strong fever, that went up to 40 degrees Celsius and I also started suffering from diarrhea, but I had no other symptoms whatsoever except for a light headache. today when I woke up I had a very strong neck pain, probably caused by the fact that I stayed in bed for 2 days straight, but I also had this annoying headache whenever I got out of bed or whenever I bend down. the neck pain is gone, fever is down to 37.7, but the headache is still there. should I worry?