In my long absence, another thing I failed to mention is that Halo and I have our first show this Saturday! Of course, that's the perfect time for her to develop a mysterious cough. The past two times I've tried to exercise her, it was cut short by intense bouts of coughing in the arena. Now that the rain has stopped, it has become dusty out there. Either she is having a reaction to the arena dust, or she has some sort of other allergy.
I took her temperature yesterday and it was 100.4, so I don't think she's sick. Her lymph nodes under her jaw are slightly swollen, but that is not unusual for her this time of year. Every spring and fall she seems to get some random lymph node swelling. I did have the vet out to look at it once, but he said it was most likely allergies.
Please cross your fingers for me and pray that my horse doesn't have an illness, or even worse, heaves. Heaves is unusual in horses under six, and her breathing didn't look as bad as the youtube videos I checked out on the condition. Today I'm going out to check on her again, and I will take her temperature. If she seems okay otherwise, I'll try working her outside the arena, and see if being away from the dust makes a difference. If worst comes to worst, we'll scratch the show. I would not feel comfortable bringing her to a strange new environment if she's not at 100%.