I tested negative for std's nine months ago and I have had the same partner for ten months. Could I have a STD that didn't show up? My dr was thorough

Std checks. The answer is almost always yes. There are 2 main reasons for this: (1) none of the tests that doctors do for sexually transmitted diseases are 100% accurate; you can miss up to 5-15% of infections. (2) you have to be absolutely certain that your partner has never been exposed, and generally, while you can feel quite confident, if you trust him/her, you cannot be 100% sure.
Define STD. There are at least a dozen diseases that can pass sexually but only about a half dozen or labeled STD's. Some are only found if you look for them specifically because only 3-4 are part of common screening. Many localities require you sign a permit specifically before testing for HIV. Not all tests are 100% accurate. That said, the treatable ones are part of routine screening.