What Causes Rapid Decay Of Teeth At The Age Of 48?

Good Evening, I am experiencing rapid tooth decay. I had a root canal less than 2 years ago (tooth 29), and the tooth is completey an apple core on the film. There was no decay back in Dec. when I had to have tooth 30 extracted due to decay under the crown. I was also told that there is leakage under tooth 31 and that it has to be extracted! I am a 48 yr old female, (4 10 126 #) with fastidious oral hygiene. I floss and brush twice a day. I have been on prednisone 20 mg 3 times since Nov for asthma control. I am also on pro-air prn before exercise ~5 X per week. My dental check-up in March showed no decay, 3 weeks later I need 2 extractions. I don t understand. ps. I rarely drink soda, have never smoked and do not drink except at Christmas. Any advice would be greatly appreciated.