he was texting his ex

My SO and I were cuddling and he gets a random text. I saw "Hey!" And then another text came up and he deleted it before I could read it. I asked "who was that?" And he said "wrong number" I walked away and came back a few minutes later and asked if he was going to tell me the truth now? Turns out it was his ex Hannah.  She texted him right after I left the night before. He put her back in his phone under "H" He said he put her in his phone under H because he wanted to forget about her. I find this bullshit. If he really wanted to forget her he wouldn't have her number. I asked what all the talked about and he said he talked about me and how he's so "happy" with me. Apparently he said he didn't want to talk to her again... if so then whyyyy did she text him the next day? I don't trust him. Any advice??