I thought he liked me, but then he posted a flirty comment on IG to another girl?

I thought this guy at the boxing gym was interested in me for that past 5-6 months. There were a lot of stares back and forth, and hovering around me. He started teaching classes several months ago, and I would attend sometimes. I am introverted shy, whereas he is more outgoing but his demeanour changes around me. He becomes bashful, smiles a lot (same with me) and our convos are therefore brief. He would continue to stare at me from across the room, and check me out on occasion on the low, give me compliments in class or after (moreso for me than the others).

For the first time ever, we ended up being paired up in class, he seemed tense for most of it, then started to warm up, smile and was a little flirty, and then was back to tense. I saw him a couple days later in a class that he wasn't teaching, and I noticed him looking over at me several times, staring, like checking on me, he seemed relaxed again. I smiled at him, he smiled back from across the room. I thought he was still interested in me but taking it slow and is shy about it (like myself). But then later that day, I noticed on the gym's social media page, he commented to another girl featured in the photo. His comment seemed jokey, possibly flirty, about how she kept being featured in the gym's bomb photos etc... He's made jokey comments to his male friends at the gym in photos but never for girls. This really threw me for a loop.