Hi,
Download, install and then run the following two apps.
The first is Malwarebytes for mobiles. This will check for and clear any malware on your phone.
https://play.google.com/store/apps/details?id=org.malwarebytes.antimalware&hl=en
This one is Avast for mobiles. This will check for and clear any viruses on your phone.
https://play.google.com/store/apps/details?id=com.avast.android.mobilesecurity&hl=en
Once you have them fully installed and updated, before you run them, disable the Data Connection setting in Settings > Wireless and Networks > more > data connection. This will turn off your internet connection. Your phone will still work as a phone but you will have no internet connection or be able to send/receive MMS messages (videos and picture texts), ordinary texts (SMS) will still work OK.
When you have cleared the problem turn the Data connection setting back on to regain internet access and MMS facilities.