It sounds like you either have a bad app running on the phone, or malware or a virus. I would suggest uninstalling anything (and deleting if was a file and not an app) that you installed/downloaded just before the problem started. If this does not fix it, you may need to do a "Hard Reset" of the device.
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the phone.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...
1. Make sure that the phone is actually powered off.
2. Remove your SD card, if there is one in the device.
3. Press the...