Hey I am a new Betta Mom myself, but I'll try to give you some info I gathered during my teething stages.
I initially had a 1 gallon tank in which I kept Bertie , my betta.I fed him thos Fish stone foods which are supposed to last a week- as advised by Petsmart ( I hate those guys, they keep bettas in small cups
What happened was that Bertie was sluggish and disinterested.
I then upgraded my tank to 2.5 gallons, got a filter, a thermometer and a small 25watt heater ( $20 for the heater and $2 for the thermometer- both got at Petsmart).
I also got Bertie a couple of fake plants and a small cave to hide in.
I then proceeded to set up the temp in his tank to 78, put a small light next to his tank and Started feeding him HIKARI BETTA BIO-GOLD pellets - he kinda spat them out for a day or two, but now he's hooked on to them and eats an average of 5 pellets a day.
The other day he blew a bubble nest and I was ecstatic.
I guess the take home lessons are - Warm temperature, Water conditionner and a whole lotta love!
Do let me know any updates and hope your fish feels better soon!