Assuming you are using an Android phone, there are a number of mobile antivirus programs you can run to check for malicious apps. I'd also turn off developer options, installations of apps from unknown sources, and make sure your phone is running stock firmware (not rooted). You can always reset the phone to the factory default and just install apps you are sure of.
Other than that there are other venues off attack where someone may be reading your emails, if you use web-based email like gmail they may have hacked/guessed your password, for example, etc. If someone has physical access to your phone that may be the culprit as well.
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