Horse riding is like riding a bike. Sure I know how to ride a bike. But to be honest, its been about 4 years since I got on my bike. Yes I know how to balance and pedal and do all that stuff, but will I be racing in the Tour De Frace? Heck no.
Your muscles are out of shape. The leg muscles you use to ride are ones that aren't used to that extreme in many othe things. Once you take a couple more lessons, your muscle "memory" will start to come back.
Also, the same goes for how to ride. You probably remember 90% of what to do when you ride, but the other 10% is gone. And again, I say give it a few lesons, and those little details will come back.
And, don't take offense that you were put in a class with beginners. Your instructor probably did that until she gets a feel of what level you are at. I'm sure in a month or two you will be moved to lessons with more advanced riders.
Welcome back to riding!!!