Lymes. Long story short, horse came up DEAD lame, suspected abscess. Next AM had a huge hard swelling on inside of stifle. Treated as a bug bite (wood wasps) and unrelated to abscess. Horse does improve, but lameness is no longer an abscess, trots a 3/5 five days later, first day he walked a 5/5 and wouldn't trot.
Treat bug bite w/Surpass, Naquasome and Bute. Horse makes a 90% improvement that day and is sound the following AM (Wed). I give him Wed off and ride Th, light trot mainly walk. Sound through weekend, walks out extremely stiff Mon AM, originally thought to be just needing to be injected. Talk to the vet, she plans on coming the next day to do a lameness exam/inject (injections had been planned pre lameness). Horse is 100% sound after an hour in the paddock, so the vet advises a light hack. Bring him in, put him in his stall, maybe 20 minutes. Come back and he walks to the x-ties happy, but his breathing is SLIGHTLY elevated. Being paranoid I take his temp, which is 104.6 You'd NEVER know this horse had a fever, completely normal, and happy.
Vet comes pulls blood and lyme titer. Still waiting for the titer but started Doxy Mon afternoon, was 100% sound Wed AM (walked out fine, no stiffness) and had no temp. Has been ridden since Friday, and seems to be on the right path (knocking on wood like crazy!).
Well the girthiness and lethargy could be from stomach ulcers. Stomach ulcers can also cause slow healing, infection, etc in other areas and systems in the body. They can lead to malnutrition and overall ickiness. When my mare first had ulcers she got scratches, that wouldn't heal, and blew up into a terrible infection overnight. Finding the ulcers at the same time were coincidence, but vet felt that explained why we couldn't clear up the scratches. Once we treated the stomach ulcers, she went back to her normal, happy, healthy self.
I'd have the vet pull a basic blood panel tomorrow and if you don't have the ability to have the horse scoped to check out the stomach, you can try a trial of something. If it's ulcers your horse will respond to the meds quickly.
It also certainly could be something like lyme, so I'd have titres pulled while the doc is pulling blood anyway.
Strong promoter of READING the entire post before responding.