A: There are probably multiple reasons for this. The first is that your head is the heaviest single part of the body (on average it's 10-12lbs) and in the sit up position it's supported solely by your neck. ouch!- especially if you don't have the vertebrae and muscles aligned into the strongest position. Secondly, a lot of people use their heads and necks to create the action of a sit up. In other words, you're doing a sit up by pulling your head up rather than engaging your abs to pull up your shoulders.
My first suggestion to avoid this is to stop doing sit ups for a little while until your abs are stronger. I highly suggest standing abdominal exercises like these:
also, try doing the ab bicycle exercise with your head on the ground. reach to your opposite knee with your hand instead of elbow to avoid pulling on the head. Another series of exercises you can try are plank-based exercises. Start in Plank from your forearms or your hands (pushup position). from here, you can do one of these options: a) balance and hold for 30 seconds and repeat, b) lift one leg at a time and balance and hold for at least 15 seconds, c) balance on one leg while you bring the other inside (under you), across (under you towards the opposite shoulder) and to the outside (to the outside of the shoulder on the same side) - then switch legs.
When you go back to doing crunches and sit ups make sure you don't have your hands ON your head. if you feel you should have them there, allow them to float behind your head. or you can cross them across your chest, or just keep them down by your hips (like in high school when they were making sure you went far enough. Was I the only one with this experience?)
Hope this helps!