What Could Cause Alopecia Areata?

Sir, I am Rahul (24 yrs). When i was 12 yrs old i was suffering with a skin problem (Lichen Planus). I had round black patches on my lower leg.I was on long medication(approx 6 to 7 yrs).At last my doctor gave injection on all the patches which cleared all the patches except one patch(little bigger compared to all and still exists). Now after all these medication, since 2 & 1/2 yrs i am suffering with Alopecia Areata. This is now leading to Alopecia Totalis. Please advise me whether this alopecia started bcoz of my previous medication history??? Thank You.