Should I stay away

For the past month and a half my fiance and I have been argueing constantly. He seems different and accuses me of everyone. He has always been open to giving me his phone to use and stopped. Ive been thinking something was up so I've been going through his phone. He has been deleting conversations.. His excuse was his phone was running slow so he cleans it up. I had his phone yesterday and a text popped up saying hi..he got very nervous and finally admitted it was his ex. He said she called a few weeks ago and he told her he was with me and very happy..my question was well why did she text back in the middle of the day on a Thurs when I'm normally not here. I broke his phone and left. Did I over react?