The very first thing you should do is stop using your oven and have it checked out by an authorized repair service.
If in fact the oven was operating with the door open, there was a failure of the door interlocks to turn the oven off and a failure of the backup system intended to permanently shut the oven off in the event the interlocks failed which, although extremely unlikely, is nonetheless possible.
The next thing you need to know is that microwave "radiation" is not the same thing as x-radiation, gamma rays, or other forms of nuclear radiation, which are referred to as "ionizing radiation" and are associated with injury such as cancer if excessive. Microwave radiation is non-ionizing radiation which, to date, has only been conclusively associated with thermal injury, i.e., injury to tissue due to heating (that's how it cooks food).
Since you were only potentially exposed for a few seconds, unless you are experiencing discomfort, you are probably all right since the temperature of your tissue was probably not raised all that much. In the early days of commercial microwave ovens, before multiple safety features were required, there were incidents where workers were exposed to multiple exposures and had some loss of muscle function due to heating damage. But those were the cumulative effects of many exposures. A few seconds of exposure is unlikely to cause that effect on you.
In any case, I urge you to stop using your oven now.
Hope this helps.