DoS Attacks launched from my java app on aws EC2 instance

I got an email from AWS saying that they limited access to my EC2 instance because a DoS attack was launched from it. I swear it wasn't me.

Basically, all I have in this EC2 instance is Tomcat7 with a java app deployed to it. The java app is a simple web app with REST calls exposed through Jersy, so nothing super fancy since I am only using it to learn web services. The java app also is connected to an RDS mysql instance to expose the database through the REST services.

I am trying to narrow down my search area to find out how this DoS attack happened from my instance. What could have caused it? what are the security measures I need to take to prevent this from happening again?