Other than emotional disturbance, what are some reasons a normally curly dog tail would remain straight?

This might sound like a flippant question, but I assure you it’s seriously freaking me out. My dog Molly is a Lab/Chow-Chow mix, and her tail is always curled. Yes, I have seen her tuck it between her legs as a frightened little puppy, but she is not in any sort of comparable situation at the moment. She’s at home with me and her dad and our roommate, and having a great time. Relaxed, happy, playing, totally normal dog in every other way. She’s even wagging it when given attention and treats, but it still won’t curl up like normal. And she seems slightly uncomfortable when sitting – she’s doing this little shuffle as she gets up.

She’s five and spayed, and in perfect health. She takes Heartguard and Frontline.

This happened in the last hour. She just went outside before that, and had plenty of time to do her business. I squeezed gently along her tail’s length to see if she yelped when I touched a certain part, but no dice. Could it be a break or a fracture even though she didn’t react to my touch? Or an anal gland or intestinal issue?

She couldn’t be messing with me more if she tried. Blood and UTIs and most anything I can deal with, but this is so weird. As you can see, I’m really worried. I’ve already resolved to take her to the vet in the morning if nothing changes. Can you guys offer any knowledge or advice about this sort of thing?