Hi, Thanks for the detailed information provided by you .
The pigmentation around the neck, thighs , breast abdomen is Pseudo acanthosis nigricans
.There is no damage of skin cells after taking contraceptives. It is related to obesity. So the best way to reduce this pigmentation is weight reduction.
Creams like tretinoin
may be used to reduce pigmentation .Tretinoin can cause hypersensitivity
reaction in the skin in the form of itching and rashes as has happened in your case.
You may stop applying tretinoin cream and apply a moderate steroid
like momatosone for a few days, this will help in reducing redness
and itching can be taken care of with an antihistaminic.
You may continue taking fish oil supplements.
After the rash disappears you may restart tretinoin gel 0.01 % for 2hours , slowly increasing the duration if no rashes appear
Hope this helps you