It seems that Jacob has lost his memory in the magical world. Some people are speculating that he did not lose them but he was only pretending. Some say that perhaps he has lost his memory but he managed to gain back all of the memories that he has lost.
It is possible that because of Queenie Goldstein, who is an expert in legilimency was able to implant all of the memories that Jacob lost so he was able to gain back all of his memories right then and there. Do remember that all of these things are just speculations. People will know for sure when the second movie is shown.