I have developed a brownish discoloration on my cheek bones this year. i have used injectable contraceptives for 15 years. could it be the reason?

Melasma?: This is a pigmentary change which affects some people- I believe people of Asian origins are more prone to it. There is medication which may minimize the pigmentation- consult a dermatologist. It is hard to say that BCP has anything to do with it however.

Multiple reason: There are multiple reasons ,but anyway you should not take injectable contraceptions for such a long time ,you need to get a dexa scan to see if you have osteoporosis if not then see your dermatitis for your cheek bone