I'm a yellow belt in shotokan karate and I've been practicing since 3.5 months..... I'm 14, male...... This is the first time I got pain in the Upper part of my backbone...... And our master said me to take rest, if back-pain occurs...... I often get pain (which lasts for 2 days) in 'waist', but he says that's good for muscle-growth......

Is it really terrible/dangerous ? Or is there any treatment/exercise for it ?