Yellow leaves????

[musicts]

Hello,
I am a fairly new orchid owner, I have three phalaenopsis orchids that I've had for about six months. They've all been doing really well, but now my little one is developing yellow leaves. It just finished flowering, and I did not cut off the flower spikes. There is a new flower spike forming. I noticed it last week when I watered them, it had dropped an older leaf which didn't really concern me, but when I picked it up to water it today a new leaf fell off, and a third is yellowing. I've been using very dilute schultz orchid food (19-31-17) once a week. The plant in question is small, probably 6-8 inches tip to tip. There have been no changes in light or in temperature. The other two do not seem to be infected! Help!!

As a side note, there doesn't seem to be a whole lot of potting medium left?