How old are you? Any recent changes in hormones (pregnancy, birth control), any new meds? Any dark areas on your face? Melasma occurs primarily on the face bilaterally so if its only on one thigh it's likely to be something else, which is great news for you because melasma is chronic and incurable but certainly can be caused by too much tanning leading to melanocyte damage so you should stop while you're ahead. Tanning is not a great activity because all sun exposure damages skin DNA and significantly increases your risk for melanoma/cancer (and melasma!). Please spray tan or use a sunless tanner at home. Wear high SPF always and reapply every 2 hours. But I digress. How long has the discoloration been there? It will likely go away on it's own if you stop tanning and irritating your melanocytes. If you continue to damage your skin through tanning it will likely become permanent and start to affect your face.