PIH refers to post-inflammatory hyperpigmentation which might have been caused due to prior any disease on your affected area.
As much as I can comprehend from your pictures, there is increased thickness on your skin too, than just pigmentation.
There could be many differentials, like acanthosis nigricans, lichen planus pigmentosus, pigmented contact dermatitis, which can be evaluated by proper history and examination.
Next Steps
Do not scratch much.
Avoid sun exposure and use sun protection daily.
Consult online for proper diagnosis and management.