Huh, seems she has trained you to take naps when she's constipated :,)

I imagine there's a vicious cycle when she has held it for a long time: she doesn't want to mess her crate, so she holds it, so she's tenser holding on, and closer to messing her crate, so anxiety rockets.

Are you treating her continuously? That is, Come! treat, attach leash, treat, move a little, treat, move a little more, treat, step through the door, treat ... you get the picture. Are you completely dressed when it's time to take her out — I fail at this a lot, and Lucy gets exasperated with me and turns off her "outside mind."