Fatigue, oh man, I hate it, and also all the various causes, I've been through a battery of tests, I'd say start with your family doc., like you mentioned, it could be any of the 3 you mentioned, I'm not familiar with TX, was there once back in the 80's, company I worked for sent me sent me to Plano for a course.
Does your area have certain seasonal plants, or even mold spores that can be causing a possible allergy?
Sometimes, things we may not have any allergy to, hits us as we get older, and sometimes not.
When I moved to PA, I had so many sinus, and other nasty respiratory stuff, after my ENT ran some bloodwork for allergies (more accurate than the skin test, Rast test?) by showing antigen levels, too complicated for my mind, I use to know this stuff, ha!ha! Well anyway, my main problem was the level of mold spores outdoors, we lived in the valley section of PA, and not many miles from where commercial mushrooms are grown in shroom houses, but the soil condition is great for growing mushrooms, even in our mulch around the shrubs.
Try some Benydryl (sp?) or some other OTC allergy med., see if it helps, if so it may be an indicator your problem is allergy related.
Still, check with your doc., today so many things can start with similar symptoms.
Hope you feel better soon

Sincerely,
DE