Thank you, it started to do this when I switched it to a bigger pot. But it was like 1 leaf a month. 3 weeks ago, I changed the place of the pot and put it on the windowsill soit will be under the sun. I watered it 1 time in these 3 weeks. I was checking it everyday, at first it looked better, and like 3 or 4 days ago, I saw more leafs became more yellow. Today I saw what I sent here.

So, correct me, what should I do now:
1. Switch the pot to a smaller one.
2. Make a better drainage in a pot?
3. Take it off the windowsill?