Strange! Do you get the heart racing feeling when the meds are actually active, or only in that time before the meds kick in? The closest I've gotten to this is that I'll often wake up to take my first dose of medicine about an hour before I really need to get up. If I'm anxious about something happening that day (work, appointment, etc) then I'll stress myself out trying to get myself to fall asleep before my meds kick in, knowing I won't get a chance to fall back asleep again, and that anxiety sort of causes a faster heart rate. But not really related to the medicine, as it's the same anxious reaction I would get back in college when I'd stay up too late studying for a test and then desperately try to fall asleep only to stay awake the whole night before the test ๐ lol
Any chance it's something like that, anxiety/stress related?