I wake up in the morning and feel a real pain in my teeth. What could it be?

Clenching/Grinding. Morning tooth pain is almost always a sign of night-time tooth-to-tooth contact. This can be either clenching or grinding (or both). This can be very painful and destructive over time. Once this behavior pattern develops, it will not stop. You need to see a dentist and have a professional nightguard made. Do this now, before there's a lot of additional damage and cost.
Many things. Pain can be caused by a number of things, including grinding, infection, cavities, or fractures. You need to evaluated locally by a dentist who can take radiographs and treat your problem.
Pain in teeth. You need to see a dentist to find out, you need a dental exam and may be xrays to find out what is the cause of the pain.