New Betta not really swimming

I recently got a new betta after my other betta died and I've been doing everything I've been doing with him that I've been doing with my other betta that is living perfectly fine. I got him saturday and on saturday and sunday he was swimming fine and was energetic. Today I came back from my classes and he wasn't really moving he was on the bottom just sitting there and occasionally comes up swims around to the top and goes back down. I'm not sure what's going on, but my other fish is doing perfectly fine. Maybe its the sudden change in weather, the weather suddenly got extremely hot today