He hasn't told about me?

Would you be worried if there was ONE certain woman your SO hasn't even mentioned his (long) relationship to, while he has no problem to tell anyone?? It's his tutor, I know about her but just met her today when I walked down the stairs and they were having a lesson. She couldnt event imagine who I am. (Btw we are both 22). I remember now that two days ago, while he was talking with her on the phone about their next lesson, I said loudly "hey babe this way" (we were out) and he got mad "be quiet wtf I'm talking! Event today he told me we could drink coffee together AFTER he finishes his lesson, I "should be sleeping till then". I don't mean he has an affair!!! But there is a reason he doesn't want her to know about me maybe?! He knew her since he was in high school and had a REALLY different life from now.