Isotretinoin for high CK levels?

The dermatologist said that he would definitely recommend and prescribe ciscutan (isotretinoin) for me (m 15), but that I would have to go for a blood test every month to have my levels checked. I go to the gym and drink protein powder, which is why my CK level is high. I told the doctor that, and he said that as long as I don't train for 5 hours a day, it shouldn't be a problem. After the first blood test, my CK level was around 380 (should be under 170). The doctor said the level was too high and that I should have another blood test in a week. The next week the level was even higher, I think a bit over 400. The doctor said that next time I shouldn't train, three days before the test. After two weeks, I went for another blood test, and I hadn't trained for five days beforehand. The CK level was over 300 again, so now I have to wait again. Why is the CK value so important and what's the point of not training for a week beforehand if I go back to training normally after I've been prescribed the medication?