Generally it's younger horses who have pain in the round pen becuase their joints are devolving, with her being 16 I wouldn't worry as much, unless she does have arthritis. It's so hard to say without actually seeing the horse, it could be any number of things. She could be sore or hot, or she could just be bored. If I were in your shoes I'd giver her a couple days off just in case she is sore, to let her rest up, then I'd take her out in the coolest part of the day, even if it means you have to get up really early before school/work etc., that way you can see how she works when she's not hot, and when you work her really try and mix it up. Don't just do the same thing every single time. Take her in the round pen and instead of immediately going to round penning do some disengaging, then mix it up some more and do something like pick up her feet, or flex her, then send her off, and try keeping her in a walk, and then get really energetic and try taking her from a walk straight to a canter, then stop and do some other kind of ground work with her, and then send her off again, try taking her from a canter to a trot, then back to a canter, stop and go. Change directions a lot. Just when she thinks she knows what to expect from you, surprise her.
Then after trying all of that stuff, if she doesn't show any change I'd call the farrier and dentist, they can have a look and it'd be good to have her trimmed and floated, and if there's nothing wrong in those areas, then call the vet. If there's nothing obviously wrong with her though, and it's just that she's bored or needs her feet trimmed, there's no since in running out and getting the vet first thing.