Well, if you mean Yamaha PSR-F51, you "feel like you don't know how to play the piano" because that is not an actual piano keyboard.
A realistic piano keyboard is a weighted keyboard (which is what the P45 provides), as it imitates all the aspects of a real piano keyboard: feeling, weight and inertia. Some keyboards even have mechanics that are exactly like those of a real piano.
So, yes, it is normal, as that is what is expected from a realistic keyboard and what piano players are used to.
It will take you some amount of time to get accustomed to it, and you need to be careful when studying and not force your fingers and wrists too much. In the end you'll probably realize that a weighted keyboard has benefits, and that "strong" response is actually useful in improving your technique and dynamics.