Definitely sounds like Ammonia poisoning. Those symptoms are exactly what happend to my goldfish and mine was really playful too. It was so weird. Try to notice the color of its gills. If they are blood red, or redder then normal I'm pretty sure you can assume that its asphyxiation from lack of oxigen. Goto Petco (or local pet store) and pick up the Quick Dip Ammonia strips made by Jungle Labs. Its a simple test and will most likely show positive in your case. If positive you need to either do a huge water change (50-75%) and/or get some sprouted bacteria from Petco which places Ammonia eating "good" bacteria in your tank immediately.
If this helps or you have any other Questions, let me know.
-Xan