Perplexed with the behavior of my male Betta fish...he seems fine one minute, and near death the next minute?

This male crowntail Betta is housed in a filtered, heated 5.5 gallon tank. He was very active and happy-acting (swimming constantly, exploring) when I first got him. I've had him about 6 months and he has grown since I've had him, so I am guessing he was not very old when I got him. About three months ago, I added a ghost shrimp and a few tiny tetras to his tank. My Betta promptly killed and ate all these critters. Then, shortly after this, he seemed to get very sick (stopped eating, swimming, etc.) I almost humanely decided to put him out of his misery, but suddenly overnight he seemed to come back to life, eating and swimming around. But, now he has shining moments and other moments he just lays on the bottom of the tank on his side. Just last night, he was playfully swimming and eating a frozen pea from my hand and acting all chipper, and now he looks dead...any ideas?