Most Helpful Guy
Yes, you should be worried. That's a very bad sign. That's a big no-no if you are in a monogamous relationship, which I am assuming you are in.
It becomes even more of a concern if you are in a long distance relationship. One of the biggest problems for those is the lack of physical intimacy. If he is getting that elsewhere, then he will be emotionally bonding with this other person more than he is emotionally bonding with you. At that point your relationship with him is dead in the water.
As crappy as the option sounds, you will most likely need to leave him and move on to someone else at this point. The longer this drags on, the harder it will get for you emotionally.
Most Helpful Girl
Shady. Dump him.
I know it sounds brash but listen to me honey: You are a beautiful, young lady and the romantic and/or sexual situations you embrace now will be setting the framework for you emotionally and spiritually for the rest of your life. Your heart is a precious treasure chest that must be protected at all cost, even the presence of others. Therefore, take no one's bullsh*t. Accept no shady, questionable situation that gives you anxiety, makes you uncomfortable, and takes away from your ability to instinctively trust a guy without hesitation.
No significant other who truly respects you, respects your feelings, and respects the fragility of your bond is going to do some sh*t like that. Point blank period. There's clearly something wrong with this guy's value system if he thinks it's acceptable to be holding hands with another female at all let alone on social media. He is shady. Get him out of your life before he disappoints you and really does something to break your heart.