Is my Betta Fish sick? Please help!!!?

Okey so i have a Betta named Prince Sascha and i bought him at petco not too long ago. I put him in spring bottled water (crystal geyser) just like petco said i could. He was very happy and active! He loved to eat too; But now it seems like he's sick. He doesnt spread out his fins like he used to. and he doesnt seem very excited to eat. He also swims around really slow, or just stays near the surface. yesterday when i fed him he swam kind of weird to go lay on the gravel. It was like a snake-like movement. This all started after i changed his water, but i used the same kind! I didnt change all the water, just around 20% ; Is it because i didnt keep his bubble nest in there? Please help!!