Nope, you’re not imagining it. When you sit for a living, you start to s-p-r-e-a-d. Not to mention, it also hurts your back, which in my view is the classic place of stress and strain, and it often derails many a client!
What can you do? Switch careers (I did!). Or failing that, just get up and move every 30 minutes. And of course, keep training!