Run Anti-spware Apps
I would run Malwarebytes and SUPERAntispyware to see if there is an infection (trojan) on your system. Then I would change my email password to something complicated like: (example) *uSP<4nine@pVT after the issue is cleared. Contact your email provider and let them know.
Weak passwords can be broken easily.
Here, and free! http://www.malwarebytes.org/ Update the definitions before you scan and fix all malware found.
and here: http://www.superantispyware.com/ Free as well, same as above.
Any problems installing, come back here for additional help.