Phal Leaves Turning Yellow/Falling Off

[Justine]

I am having a problem with one of my phals. 3 of the upper leaves have begun to turn yellow and drop off. They are the newest leaves on the plant, so I know it can't be good! I have read that yellow leaves can mean that the orchid is getting too much light; however, I have 2 other orchids sitting in the same window with the "sick" one and they both have thick, green, healthy leaves. I also read that yellow leaves could mean the plant is getting too much water, or that it has a disease.

My question to you all is... what are some solutions to these problems? I have no idea what kind of disease it could be and how I would fix it.