You say it's in your back, but might it be in your buttocks? You're describing a pinching of the sciatic nerve. It can be from the back or the hips, but if it's centered in your buttocks it's the periformis, a small but, literally, pain in the butt muscle that the sciatic nerve runs very close to in most people. When it's inflamed, it often pinches the sciatic nerve. Obviously, you started out on a program that wasn't right for a beginner, and squats and steppers aren't beginner exercises if done intensely. Now, you probably had something already going on in those areas -- if you're someone who sits regularly, uses a computer regularly, drives regularly, the hunched over position required to do these things compresses the spine and makes these injuries more likely. If I were you I'd probably, at this point, go to a doctor and get a referral to a physical therapist who can show you stretches and strengthening exercises to hopefully teach you how to fix what's broken and protect it from breaking again, especially since you've caught it early. I have really bad problems with this. Originally, I got it from riding a bike, which is a common cause, and my knee while running one day, but as someone who's been athletic all my life it was probably coming at some point. Sorry this happened to you -- personal trainers aren't always that well educated -- some gyms will have a biography of their personal trainers on the wall somewhere and you can tell who has studied the most about movement. But if you ask for something, they will give you that something, so be careful what you ask for.
You're repeating the same question that we replied to about 3+ weeks ago!