Is it unusual to titrate up to a particular dose and do well for a while only to have side effects come back and have to go down in dose?

I've been placed on Strattera and was titrated up to 80 mg. I stayed at 80 mg for two weeks and then out of nowhere, I started feeling very jittery and anxious. I really felt like I was going to come out of my skin. My pdoc has reduced the dose down to 60 mg and so far, so good but I don't understand how I could have been on 80 mg for two weeks and everything was fine and then start feeling like hell seemingly out of nowhere.

Anyone heard of this or experienced it?