Try the following in case there is a virus or malware installed in the phone
Download and install an anti virus app- example only
Download and install an anti malware app -example only
When you have installed both the apps, re-start the phone in ''safe mode'' and then locate the AV app in the apps list and run it. Follow the prompts if it finds any viruses.
Once the AV app has finished, find and run the anti malware app and do the same. Hopefully between the both of them they will clear your system back to good health.
Have you installed any new apps lately?