It might be a SQL injection attacks that alters your data.

Here are some important suggestions to keep your site secure:

• Install SSL on your main server and if you are using CDNs make sure to serve the ssl from your server rather than CDN's ssl
• You can activate WAF (Web Application Firewall) to prevent layer-7 attacks like; XSS, SQL Injection, Cookie poisoning etc.
• Use a CDN that provide protection against DDoS
• Hide and secure your website admin panel with 2-step or IP based access
• Make sure your Coding & Design standards are up to date
• Up-to-date your application, plugins and server
• Keep regular backup of your data

You can also follow this guide on how to secure a website.