NoID Dend with black and yellow spots :(

[LoveTheLittleOnes]

Got this one 2 months ago, it was in full bloom so I decided not to repot. Then it shot out 2 lovely new canes which have been growing rapidly, so again I put off the re-pot (the plastic pot is just sitting in the ceramic one). It was in a south facing window with high light for 3 hours of the day (the rest of the time it's shaded), but it does get quite hot in the sill. I noticed a new leaf yellowing so I moved it away from the window 2 weeks ago. The leaf has since died and dropped off, and overall its getting worse... I'm hoping that I've just been abusing it with too much light or mineral deficiency and that its not a virus… or fungal rot? I'd really appreciate some feedback! Worried about anything spreading to the rest of my plants
Thank you in advance for any input!!