I'm getting a fowl odor coming from the bathroom sink.
The septic tank was pumped out three months ago.
There's no odor from any other drain in the house.

Check that there is water in the trap, that U shaped pipe under the sink drain. It may be leaking water and thus not sealing the drain from sewer gases. There should also be a clear vent of that part of the drain system that may be blocked.

If your trap is working that odor probably isn't sewer gas but it could be roting hair and body oil. Try mixing 1/2 cup table salt with 1/2 cup baking soda and dump in the drain, now follow with a cup of vinegar. Tomoorw flush with a bucket of very hot water.