Stood in front of microwave with the door open

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.

When reading your post, it seemed to me that the most likely explanation for your experience is that the microwave did turn off, but the ventilation did not. You may have mistaken this for the microwave not turning off.

If indeed the microwave didn't turn off, you would have felt pain and other sensations immediately. Get a new microwave in that case, and throw out the old one.

Edit: With other effects, I was referring to the discoveries of Allan Frey. Also, being exposed directly to a couple of hundreds of Watts is a different story than your local wifi. The heat will be felt immediately. The frequencies are also selected to penetrate food including meat and human tissue. This would be very unpleasant soon.

I mentioned this in a comment, but will post as an answer as well.

Apparently, if you run it long enough, some microwaves will keep the fan running for cooling even though the microwave is no longer producing microwaves. My last roommate's microwave did this at the 2 minute mark, and even though the microwave was done cooking, the running fan made it sound like it still was running.

You should be able to confirm the behavior by looking in the owner's manual that came with it (or looking up the manual for your model online). Alternatively, the behavior may be described on one of the stickers on the microwave itself (which was the case with my former roommate's microwave). The sticker may be on the back of the machine, or it may be placed along the frame hidden by the door when it's closed.