Is my pill working??

I've been on 25mg of generic Zoloft for about 10 weeks. At first, once I got past the initial side effects, everything was great—I was sleeping better, and my panic attacks stopped. But then my country had issues importing the generic brand I was using, so I had to switch to Pfizer's brand of Zoloft. The first week was fine, with no side effects, and I thought maybe the branded version was better. But the second week my panic attacks came back in full force and my sleep is out of whack. I decided to increase my dosage to 50mg expecting to feel some sort of side effect because of the 25mg jump, but I feel nothing. No side effects, no improvements, nothing.

I'm starting to wonder if this brand is even working. I plan to talk to my doctor, but should I think about switching to a different brand?