My Spider BP gets very tense when I first take him out because he's normally a nervous guy. I have another idea of what you mean by stiff because I've seen Kingsnakes and Milksnakes do that before,

Although the closest thing I could really think of similarly is that your snake tenses up right after you take him out because he gets a little scared.

About the lower body thing, a lot of snakes will swing around their lower ends of their bodies because they don't feel supported enough in that area. So if it's hanging down they might swing around a little bit because they don't feel supported in that area of their body.

It would help to see a video and show us what you're talking about, but if he's fine and eats & drinks normally then I'm sure he's okay and doesn't have IBD.