I've never used Flubenol but as it's for parasites (such as flukes) I think you are on the right track. I know you said you can't see any white dots - but flukes can be internal as well as external. Gill or body flukes are very hard to see, anyway - you have to look at the fins through light and you sometimes see a vague, whitish dot - sometimes only one or two will be visible to the eye. In your second pic down, I can see one on the fin, quite clearly.
Yes, it is quite possible that a parasitic infection has lowered its immune system, causing a bacterial infection too. This is often the way, where they start off with one thing and if it's not treated correctly and cured it then can snowball into something else.
Maybe do as Wilder suggests and use an anti-bacterial med along with the Flubenol?
Either way, hope your fish recovers soon!
Athena