I have extreme lower back pain?

yes go see a doc, preferably a chiropractic doctor but a conventional MD would be good too and don't crack your own back, it actually does alot more harm than good. you might have herniated a disc or other subluxation issue plus inflammation. Large can irritate a womans back. Make sure you are wearing the proper bra too. Go get refitted at victorias secret or nordstroms about every 6 months.