No, you are not over reacting. Your worries are normal.
But before jumping to any conclusions, few questions. Have you met any of these girls? Are you also friends with them? If you wanted to be friends, would he be ok with it? Have you ever read these texts? Are they flirtatious in nature?
Ask yourself those questions .. If he is flirting with them, if he doesn't want you to know them, if he is hiding what's going on with those girls, then you should be worried.
5 years is a long time to be together. I hope you have developed enough trust and communication to address this issue head on with him.