I'm so sorry to hear that. :( I would definitely encourage you to contact Customer Support so that our representatives can investigate. This is a great article that will walk you through everything you need to do (including what steps you can take to help secure your account): https://us.battle.net/support/en/article/compromised-diablo-iii-account

If we can, we'll help out. In the meantime, consider a Battle.net Authenticator or SMS Protect as additional security options. You can learn a bit more about them here:

https://us.battle.net/support/en/article/battle-net-authenticator-faq

https://us.battle.net/support/en/article/battlenet-sms-protect