Right, before an expemsive vets trip I would look out for vent pecking!
Basically one of your others might be attacking her vent area with it's beak. There are some potions you can put on the area that the others would not like to taste if this is the case. Look out for anyone who might be trying to "bully her" to get up the rankings. You may not see it going on as it's usually a night-time thing. However if you see one having a go at her during the daylight hours, try seperating it from the flock for a few days to a week and see if this helps the condition.
In rare cases they can literally rip out anothers insides and cause them to die (although I've not seen a case quite so bad).
I'm not saying it's definitely this, but it sounds likely especially with her not wanting to come outside in a hurry in the morning!!
It's quite a common occurance, but obviously you don't want it to go too far!
Failing that, is she perching anywhere with a slightly rougher / sharper corner to it than the others? Could just be a bit of "belly rub" as I call it. If you're using something along the lines of 2x4 for perches try replacing it with wooden broom handles (about a quid each from wilko's)
ABOVE ALL, if she is happy and laying well I wouldn't think it's cause enough for concern to bother wasting your money at the vets with it. Remember these girls had rough innings at battery farms, and some had to fight for their feed. It can become habit to some. Keep them closed away from the others (seperate pen in same area) so that they can see but not have any contact - a week in confinement like this usually does the trick
Hope some of this info helps