I'd definitely agree with issacNeedsHelp about it being a cystic or nodular lesion. These can have an annoyingly long healing cycle. Essentially what a nodule/cyst is is when the infection in the pore grows large enough that it ruptures the bottom of the follicle, letting white blood cells, plasma and other immuno-response mechanisms rush into your pore. Good news is this is your body fighting the pimple; bad news is that this causes major inflammation and redness (not to mention a soreness underneath the pimple). At this point the rupture is still open, and inflammation is still occurring as your skin heals itself. When the rupture heals itself, the WBC's will kill the infection pretty quickly.
Where does the accutane come into play? Your getting less acne elsewhere because the isotretinoin is regulating the oil production in your pores. This is super beneficial as excess oil lets the bacteria in the infection multiply easier. But for this one nodule, decreased oil isn't the only thing your skin needs. It also needs to heal the ruptured follicle, which can take a decent amount of time. Your skin will continue to shed, which will push the lesion to the surface and mend the rupture under it.
The best thing you can do is leave it alone. I wouldn't recommend putting anything on it, unless you're sure it won't further irritate it. Make sure you don't touch or pick at it, this will just keep that rupture...ruptured. The redness is just hyper-pigmentation from your acne beforehand. This should subdue to normal over the next 6-12 months.
The moisturizer is a great call, keep applying it (avoiding the nodule/cyst). Once you're done with accutane, I see a lot of people here who begin a benzoyl peroxide regimen after isotretinoin. I'd definitely ask the doctor who prescribed you the accutane what he/she thinks is best.
Hope that was informative. Looking at your pictures it looks like you don't have too much "active acne" left, so congrats on that...I'm sure that's an improvement. Good luck getting clear!