I have planted a mango seed and it was growing fine until a few days ago I noticed that one of the leaves had started going blackish. When I touched it, it fell. Yesterday I noticed that another one had started looking the same as the first one and today I found a third one. The last two are yet to fall off.
A bit more context: it stays in my room and is at room temperature next to the heating and right under the window, I try to moisture it and water it between one and two times a week, so I doubt it has anything being overwatered. I am afraid it might be sick and have no idea what to do. Should I change its soil?
I have also had another mango for an year and a half that I gave away but it has never had such problems.
Edit: I have also noticed that the lines on the upper leaves have started going black.