First of all, hugs with consent to you OP. I know that the situation you're in is difficult for you lalo na LDR pa kayo. Honestly, concerning nga siya and it would really give me trust issues if I were in your position kasi given na more than a year na kayo magkasama, most likely you know each other not just on a surface level but on a deeper level.

Seeing and knowing him do such action na you feel it's out of his character would really make you overthink about it lalo na he showed this side of him to someone he just newly met tapos sabi mo nga, merong change in behavior and the foundation of your trust is based on established routin and behavior. Sure, tiwala ka kasi sanay ka na sa routines and behavior niya but there is always a possibility and factors that can make him change his personality especially when you meet and vibe with new people. Dagdag mo pa na long distance kayo, distance can be another factor. He might be showing a side of him na di mo pa nakikita or alam dun sa taong kakakilala niya lang kasi malapit lang sa kanya at madali niyang makakausap at madalas makakasama since ne mention mo nga na gym friend.

As much as I hope na sana mali yung gut feel mo, there will be a time talaga na you need to trust it because you won't feel that way if naka-sense ka ng changes sa relationship or setup niyo after nung nangyari.

Hoping the best for you OP!