I had a similar thing happen to two of my clowns. The first one died then the second one got infected. But the second one eventually recovered after 2 weeks or so...
I dont know what it is either but if there is any hope for the fish, you should put it in a QT asap and make sure you look over the QT consistently. Daily if you can.
I dont think its an oxygen issue, if i had to guess, i would say its internal.