You know, some people remain on the same brand/dosage and get several pronounced sheds during this time, maybe you're worryin about something that would've happen anyway..?
I moved to generics and researched the whole thing a lot before doing so, I've not heard of Teva USA (is that right?) I read a lot on Cipla + Dr. Reddy's brands. I use Cipla Fincar now and no obvious difference with my reaction to it. So much cheaper though it's silly, I moved up dose at the same time.
Does the stuff you ordered have a batch number etc? Medical notes etc. Mine had all of that, I had no doubt that it's legit.. well lets hope it is!
Even if not I think the idea of duff meds, or finasteride at least is quite unlikely, as I'm pretty sure it's dirt cheap to produce, so theres little to no point of passing something else off as it, thats just my opinion though.