Hm. Fish hanging out at the bottom of the tank

And I'm deeply paranoid.

Housing
What size is your tank? 3g
What temperature is your tank? 78 F
Does your tank have a filter? Yes
Does your tank have an air stone or other type of aeration? No
Is your tank heated? Yes - new Hydor Theo 25W
What tank mates does your betta fish live with? None

Food
What type of food do you feed your betta fish? Hikari Betta Bio-Gold pellets and occasional freeze-dried bloodworms.
How often do you feed your betta fish? Twice a day with fasting on Saturday and Sunday.

Maintenance
How often do you perform a water change? Partial changes 2x weekly and substrate vacuum 1x weekly
What percentage of the water do you change when you perform a water change? 1 25% change, 1 50% change
What type of additives do you add to the water when you perform a water change? API Stress Coat+

Water Parameters:
Have you tested your water? If so, what are the following parameters? About 2 hours ago

Ammonia: 0ppm
Nitrite: 0ppm
Nitrate: 5ppm
pH: 7.2
Hardness: ?
Alkalinity: ?

Symptoms and Treatment
How has your betta fish's appearance changed? It hasn't
How has your betta fish's behavior changed? He's spent much of today hanging out at the bottom of his tank or behind his heater, or on top of his filter. Then he swims up, hangs out in his plants, gets some air, swims a lap or two, then back down to the bottom. Eating normally.
When did you start noticing the symptoms? This morning, when I got to work.
Have you started treating your fish? If so, how? I tested his water parameters and everything was normal. Did about a 1/3 water change anyway and vacuumed his rocks, and added a little extra Stress Coat. At this very moment, it kind of looks like he's trying to bury himself in his river rocks. Freakshow. He's swimming into them, face down, like he's hunting for food or something. I have no idea.
Does your fish have any history of being ill? Not in the three weeks I've had him, no.
How old is your fish (approximately)? 4-6 months? Maybe?