Why would stimulants cause relaxation and sleepiness?

I have severe anxiety, ocd, and very severe fatigue b/c of CFS but I've tried all stimulants and they wipe me out, bring my heart rate down, and send me to bed. I just took exedrin migraine which has caffeine and my heart rate went from 130bpm to 80 and my anxiety calmed down but I'm severely sleepy, severe. I know that people with ADHD have this reaction, are there other reasons?