My betta has been lying on the bottom of his tank off and on for 2 days, whats wrong with him?

I have had this fish since around October. He is in a 5 gallon tank with a filter and heater. He usually likes to hang out on the suction cup of the heater. I have tested the water and it is fine. For the last few days he has been lying on the gravel, or under a little bridge and his gills seem to be really pumping (for lack of a better word). He has never acted like this before. When I open the top he still swims up to see me but then quickly goes back down to the bottom. I have put some "Bettafix" in there but Im not sure what else to do....what could be wrong with him?