my bf’s ex texting him

i was on my boyfriend’s phone the other day and saw that his “ex” and him were talking. he told me earlier in the year that she texted him wanting to become friends again, but i don’t trust her at all. she’s showed many times that she isn’t over him and of the couple messages i saw she was trying to make me look like a horrible person. also he always says that they didn’t really date, at least not to him. he knows i don’t like her at all and i’m confused as to why he hasn’t mentioned that her and him have been talking a lot more recently, because he always updated me when she contacted him. it makes me uneasy and i don’t think he knows that i saw that they text again. i don’t know what to do, advice ?