Well the pill is certainly not causes it so I would g forget about that. It is probably just the hyper pigmentation left over from your acne clearing up from the pill. It may take some time to go away but it will fade eventually, but do not stop taking that pill if it cleared your acne.

What pill are you taking? I've heard of people having pigmentation issues using some brands of contraceptive but perhaps you could switch pills if that's an option for you? I've heard ortho tri cyclen lo is pretty good at controlling hormonal acne. the other thing i would suggest considering is starting Daniel Kern's regimen and stopping your pill soon after. I've had great results with DK's regimen and I think it would be especially beneficial if you're already clear now to take preventative measures by starting the regimen. finally, you can ask your derm to put you on antibiotics that control hormonal acne without the added side effects of pigmentation.