Hmm....it really looks to me like it got caught in the filter. You can go get these round filter sponge things, cut one to size, and stick it over the intake so it doesn't happen again.....or at least to eliminate that being a possibility, if it ever happens again, or gets worse.
For right now though, dissolve and add some Aquarium salt to his water(1/2 teaspoon per gallon I believe...) and up the water changes to keep his water extra clean. Also, remember, salt doesn't evaporate, so only add salt equivalent to the amount of water you took out ;)
Regardless of what it was, that should help prevent infection and finrot from setting in :)
Oh, and BTW....if you have a filter, your tank will cycle so long as there is a source of ammonia(your betta in this case).
Heres a little thread that explains cycling
And Here is one that explains how to do a fish-in cycle
When was that 100% change? How long did you have the tank set up before that?