See a dentist. This tooth may be dying and need a root canal. Also trauma may be the reason the tooth is moving. The dentist should be able to tell why the tooth is moving with an exam and a radiograph.
See a dentist! From the little information given, it sounds either gum disease or that tooth had an abscess. Your best option is to see a dentist, get a complete evaluation of your dental problem and possible treatments. The answers are best given by someone who sees you in person. Don't delay & Good luck!