The panic attack was probably due to a snowball effect of you starting to notice side effects of a drug your not familiar with. Once you saw one side, then another, and another eventually leading to increased anxiety leading to panic. Now that your brain knows you didn't drop over dead the panic and anxiety should subside.
You have to realize though that as Magnus I believe said any med, prescription or OTC, can certainly cause Anxiety especially when your messing with hormone levels.
I have a feeling you'll be fine now. If the sides continue for more than a few days I'd just switch to something else. You have plenty of options....AR....