My horse's left side is bothering him...

I'm a little worried because my 12 year old gelding seems to be bothered whenever I touch his left side. When I go to pet him or approach him from the right he is fine, normal, happy, behavior, but when I go to his left side he pins his ears and acts very annoyed. I've never seen him react so strongly. The other day he wouldn't even let me mount up. He's walking fine and doesn't seem to be in pain. Any suggestions or any ideas to what this is?
Thanks for the help

Especially if it's new behaviour I'd say it is almost definitely pain related. If he seems otherwise fine, not lame, etc, who knows, but I'd want to get him checked out for ulcers, cracked ribs, etc. Have you tried mounting him from the right? How is he with saddling/girthing from the right? If OK, I'd think something like gastric ulcers is more likely than actual injury.

Thank you for the help. He's fine with cinching, its only is when I'm mounting or touching him on the left side. Almost as if he doesn't want me to ride. How urgent is am ulcer? I've never had to deal with one before. I'll be out of town for two days, should I have a vet come while I'm gone?
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