Do I have a right to ask my boyfriend why he is texting his exgirlfriend?

His exgirlfriend has moved on and he was once obsessed with this woman. On Thanksgiving he texted her asking her how she's doing and called her by An old pet name. I found out about this due to an overage in minutes used on our shared cell account. I asked him why he texted her and he told me that he want going to answer that and he was mad I knew about it. Then he went into his phone and deleted all the messages. I thought that was sorta a strange thing to do if it was innocent contact, why would he get so mad and delete messages, them make me feel like crap about myself the rest of the night by saying some hurtful thing.