That could be from an abdominal weakness because your abs are working extra hard to push against that belt so that you can maintain a flat back and good posture in the torso.
I would try not to wear the belt as much; maybe only when you get up to 80% or so. Also, it may be a little too tight causing your muscles to strain even more when trying to contract.
Just some thoughts. But definitely do some core work to keep your torso strong so you're not relying completely on the belt to achieve good positioning.