Sometimes riding on a very wet day can result in your bike wheel becoming water logged. The wheel has been designed to let small amounts of water out, but if there is a lot it can get into a different compartment.
When water gets into the rim, the first thing to do is take the air out of the tire. Once the tire and rim strip are removed, the rest of the water will come out of the rim or spoke holes. Spin it around in your hands a few times to get it all out, let it air dry, then put it back on.