Hi,
It is likely that you have developed irritant contact dermatitis to the tretinoin. This is common if higher concentrations are used or application has been too excessive or too frequent.
Strategies to manage this include:
1. Stop application tretinoin for the next 1-2 weeks
2. Apply a bland moisturizer at least twice daily to the affected area.
3. Avoid use of abrasive cleanser.
4. Avoid usual cosmetics for next 1-2 weeks.
5. Avoid other potential aggravating factors such as sun-exposure, heat and sweating.
Usually, the skin irritation will subside quickly if the above strategies are employed.
Hope this helps!