Boyfriend flirts at work found conserning text message... opinion?

[ASINGLEMOM]

My 36 year old boyfriend is in retail sales. Attractive looks young, flirts with girls at work. One girl in particular is 20 years old he claims is like a little sister yet he has a text sent to her where he calls her sexy and stuff. That doesn't bother me too much (flirting is flirting) anyway, she sent him a text saying she's so disappointed that he doesn't want to hang out with her due to his having plans (with me) that night. He claims it's because he told her that he had been going out every night and planned on going out that night he got that text. He said she didn't try to make plans and that she's a drama queen and that she has a boyfriend. He also said she's not even old enough to drink. Thing with that is that he's dated girls under 21 before he met me, and I'm not thin by any means. Should I be concerned? Am I just being insecure? Also, he's pissed I got in his phone, is it really that big of a deal?