why does the skin looks thin whe using retin-a?

Retin A (tretinoin) is a Vitamin A cream used to treat acne. The cream can make your skin very sensitive to sunburn and can initially make your skin look worse (inflammed) before it gets better. You should continue with treatment if tolerable. It can take weeks or months before an improvement is noticed. Your skin should not look thin. If it looks discoloured you should see your doctor. You will find some useful information here.
http://www.drugs.com/mtm/tretinoin-topical.html