Hi. Greetings. Sun tanning can occur even with short exposures and this is a protective mechanism of skin which is called immediate pigment darkening. Don't worry, continue to use a sunscreen of high SPF at least around 30 - and reapply every three or four hours if exposed to sunlight for long duration. The following precautions would help you :
1. sunscreen should be applied half hour before sun exposure
2. Daily bed time use of moisturiser like cetaphil or emoderm in case of dry skin. Reapply during lunch break during office hours.
3. Use Peeling agents prescribed by dermatologist for reducing tan. Chemical peels and micro derm abrasion is also an option.
4. Evaluation by dermatologist as to rule out any other causes of skin pigmentation.
5. Kindly read my blog on daily skin care on Practo health feed.
Hope this helps.
Warm regards. Dr Govind S Mittal, Excel Care Hospital, Bangalore.