AWS giant data transfer

I have a linux and windows instance on amazon ec2 for around 7-8 months. Every month i had billing 0.01$ - 0.06$. But two weeks ago i received an abuse report. I looked to my billing and it was of around 20$! 98% of it was DataTransfer. I terminated my instace and changed elastic ip adress. Everything seems good utill today my billing was 31.95! On picture you can see giant month data transfer out beyond the global free tier. It's around 3.5 TB.

Look down for additional info

Was it DDoS? Will IP changing help? Can i setup a data transfer limits?

P.S. Sorry for my english.