This sounds like it may be a case of phishing. Are you using the web, desktop or iOS wallet? The first thing you should do is create a new wallet and transfer all remaining funds to the new wallet.
Secondly, you should switch to the desktop wallet if you are storing higher amounts of XMR as all web wallets come with a certain level of risk.
Please be sure to store your login information very safely and off your computer entirely. When navigating to MyMonero.com, please be sure to type out the entire address as phishing sites may appear as an autofill suggestion with only minor url differences.
Always make sure that your computer is malware free and that you recognise and trust all applications and web browsers you have installed. Do not ever share your private login information with anyone else to ensure you are as well protected as you can be.