Is this happening immediately after a
water change, or does it happen throughout the day?
Shawnie is correct that plants produce oxygen during the day, it's a byproduct of photosynthesis. It is possible to have enough photosynthetic activity taking place in the tank that the water has become saturated with dissolved oxygen to the point where it can hold no more. If you get to that point, any further O2 produced appears as small bubbles, an effect is commonly called "pearling". You can read more about it on various aquatic plant forums (plantedtank.net, aquaticplantcentral.com, etc.), or
here under "Pearling and Lost CO2".
In general, to see pearling, you need a fair amount of lighting and CO2 present to encourage the plants to produce that much oxygen. If you don't have good lighting and a steady supply of carbon for the plants, this is not likely (though still possible).