Does egg laying make them hotter?!?

Okay, I know this is going to sound like a weird question, but one of our three 9 month old Coronation Sussex finally starting laying last week (the other 2 free loaders....that's another story).

Anyway, she seems to be an entirely different chicken now. Much more desperate for food (seems like that would be normal), more of the leader when she was a follower before and here's the really weird one: she seems to pant when it isn't even all that hot (the others are not panting).

I about freaked out when I noticed and immediately started them all on tetracycline for the last 5 days. I lost a bird about a month ago to pneumonia (vet couldn't determine if viral or bacterial in nature) so I thought I would be aggressive just in case because I didn't want to lose another girl. They've also been getting treated for northern fowl mite with eprinex pour-on and had a dose on Saturday. I think we may finally be winning that war.

So, I'm starting to think maybe that a big, fat girl like my 8+ lb sussex may just get a little hotter than everyone else with that egg factory she's got going now. The panting has been noticeable during the hotter times of the day (i.e. afternoons vs. mornings and evenings) and seems to be increasing as the temperatures have been getting a little hotter each day this week.

I've seen no improvement from the tetracycline so I'm at the point of stopping that. Also, I would think that if it were gape worm the multiple doses of eprinex that she has been givin would have taken care of that. I've read some posts that say eprinex is effective for gape worm and some that say it is not, so I'm not sure. She hasn't been displaying any other signs of gape worm (lengthening her neck or lethargy). Actually, she is quite active and will sprint (well, more like waddle-run) across the yard if she sees me come outside because I may be about to give her a treat.

Any thoughts? Has anyone else experienced an unusually "hot" layer?