Nearly every time after feeding my betta, he will be unable to swim down, and floats kinda sideways.

The issue goes away after around 12h of fasting, but seems to reoccur even after 4 days of fasting. I feed him 2x high quality betta pellets aroud 0.5-1 millimeter in size 2x a day, with 1 fasting day per week. Any ideas what might be the cause?