Does Armour Thyroid cause some initial symptoms which subsides later? I'm experiencing a little dizziness and a mild headache. Dosage : 15mg * 2 daily.

You will NEVER. Be balanced w/armour thyroid as it's ground up pig and not standardized on the basis of thyroid content. You can't check levels with available tests at the lab. It contains T3 (liothyronine) which can be dangerous. Use only brand name T4 replacement, it's better and safer.
Not likely. It isn't likely that the symptoms are from such a low dose of Armour Thyroid in a short time. I don't have a good reason for the complaint.