Eggbound or broody?

Another day another worry.  One of my ex-batts is not coming out of the nest box.  She was in there for a couple of hours yesterday and continually scratching the nest box floor, like she was trying to get comfortable.  She was panting(!), can't think of how else to describe it, and no noise coming out.  I encouraged her out and she was fine for the rest of the day.  Now today it's the same thing again.  I was worried about her being egg bound?  Or is she broody?

I will try and feel her undercarriage when I get home this evening.  Will try the calcium tablet, how much does she need to drink to have an effect and how soon will it work?  Will also try the warm bath.  Do I just hold her in it?  Will it still work if she tenses up because she's not that used to being held. 
Due to my obvious in-experience (see above), I think the vets is probably the best plan.  How soon will it get serious, should I try to take her this evening, or give her a bit longer.
Thanks for the advice, she is my favourite hen (if you are allowed to have favourites).

Many many thanks for the advice.  I will leave work earlier enough to take her to the vets, if she still looks like she's having problems.