if you accessed your personal email on your company computer, then I think you kind of left that door open.
If he cared to be bothered to look, then he did. You left him with the opportunity.
NEVER EVER access personal information on a company computer!
You want to keep it all separate. Time and time again, people just merge it all together, and it comes back to bite them in the behind.
Change your password for starters, and stop accessing your personal email at work on their computer.
And by the way, if you talk on the company phone, you have no idea, but they could also being recording that as well.
Personally, if I were you, I would change my password, I would not look at my email on company time, and just keep on moving forward with my job.