That has actually happened to me several times. Plus I don't always have any mobile security installed on my phones because I don't really see the need for those. The one thing I usually do and I will recommend as well, is for you to backup your files, including your media files to your PC, and then do a factory reset of your phone. If you actually downloaded an unknown file, then check the download section of your file to locate it. Another thing you can do is to install a mobile security app from the app store, which might really help you to flush out the virus, or malware, etc.
No need to panic.