Yes, my back aches a bit. As Meldy says, it's because your wetsuit makes you buoyant so your position in the water is different. Your legs are probably higher which makes your back curve more.
I'm not sure what you can do about it though. Try a wetsuit with thinner legs (so less buoyant)? Swim in OW more so your back gets stronger? Improve your body position in the pool so you don't have such a difference when you get in OW?