There are nerves near your spine that can have pressure put on them when you curve your back the wrong way. If you have been wearing high heels, that can cause it, or if you sit in a chair that puts your back in the wrong position. If you carry heavy school books, that also hurts your back.
If you have a bag of corn or peas in the freezer, that makes a good ice pack. Sit in a better chair and put the ice pack on the spot that hurts. Keep it there for a good 15 minutes, and it will help. Make sure you wear good shoes that support your feet, and don't wear high heels or flats with no support or sole that helps absorb the shock from the floor. Walking is the best exercise for back issues- but only if you're wearing the right shoes. The best are Dansko Professional clogs, Merrills, Ecco or Mephisto, but a good sneaker with an arch support (NOT Keds or Chucks) is fine too.
If you can take some Tylenol or ibuprofen, that will help- but don't use that as the only thing you're doing for it. Make sure you get in a couple of 15 minute walks each day, to strengthen your lower back. If you still have problems, talk to your doctor and get a referral to an orthopedist- and don't EVER go to a chiropractor. They can actually do more harm than good. Go to an orthopedic specialist- they may give you some physical therapy visits,which can also help. SO,bythe way, can yoga.