I have to agree with the other posts, fill out the questionair about your fish and try posting this in the category about diseases and emergency.
From just reading what you've said I would first suggest upping your water changes, I'm not completely certain what would be recommended but I'm pretty positive that if your only doing 1 100% water change on an unfiltered 2 gallon with a betta and frog in it than the ammonia levels would be really high.
Also I would HIGHLY suggest getting a new tank as soon as possible, a 2 gallon is kinda small for a betta OR frog alone, but both together is way too cramped. Most people recommend a 2.5 gallon bowl minimum for a single betta.
peace. love. & bettas.