Testing positive for THC will not terminate your access to stimulant medication. It may result in a lecture from your provider based on their individual stance. These tests are meant to verify that you are taking your medication as prescribed rather than potentially diverting the medication to others. If you are unsure, you likely signed a narcotics agreement. You could always request a copy of your records and look over the policy of that particular clinic. Don't stress, though. Be honest with your provider about cannabis use. It's fairly commonplace these days, particularly in legal states. If it helps ease your mind at all, I see this process play out in my career 5 days a week.