Just starting W Prozac. Is this normal?

Three days ago, I started taking Prozac 20 mg. I've never taken any psychiatric medication before. I was prescribed it because I've been having anxiety attacks. The thing is, I woke up today feeling like I have brain fog. It's similar to how you feel the day after smoking a lot of weed, like it's harder to concentrate. Has anyone else had a similar experience? It seems strange to me, and I'm not sure if it's the medication because I was told it takes four weeks or more to start working.