First of congratulations for the 21 days or now more and there is still way to go before you may feel safe from re use to a certain extent of so called self control and making the right choices. The wisest you already made, that is to quit but you got to keep working hard to stay quit for all times to come.
Your body is on its way to healing. And during the process of healing the system may over heat as it is cleansing your body from the toxins and chemicals that have found their home inside you. SO, no need to worry. JUst take enough fluids, fruits, especially oranges. And a multivitamin plus the balanced diet. Exercise is going to benefit you greatly, especially a walk in the morning on a regular basis. You will feel coming back to life more and more with exercise and realize the beauty of truths for yourself.
Take care
Work out for a few hours and see if that doesn't really help.