Good morning - I'm Dr. Sara. I'm a licensed veterinarian who works exclusively with dogs and cats. I'm sorry to hear that Daisy has been off lately. I also apologize for the delay in our response. I realize that you may not need our advice anymore, but I just got online for the morning and I don't want to leave you hanging.
While it's possible that the melatonin isn't agreeing with her, twitching isn't a sign that I typically expect from melatonin. Sleepiness is something I do see frequently with this medication. In any case, to be safe, I'd stop the melatonin for a few days and see if this helps.
Muscle twitching can also be sign of pain or discomfort, anxiety, neurologic issues like partial seizures, or metabolic problems like low blood sugar or toxin ingestions (chemicals, medications, flea products, etc). Try to think back to what she may have been exposed to within the 12 hours before the signs started. If she's eating and drinking and acting OK otherwise it may be OK to monitor her, however if the twitching is still going on, I'd feel more comfortable if she saw a veterinarian today for a physical examination. I hope you can understand - there's such a long list of things that could be going on that I can't sort it out without actually getting my hands on her.
I hope that she's feeling better this morning, but if not, I'd give your vet a call. You can absolutely reply here with more questions or an update - we can chat until I've covered all of your concerns.