Labmom,
This has also happened to me. Just this past Wednesday I had to have .5cc taken out. My last fill was Oct '08 and all of a sudden in Feb I started noticing that I was getting tighter and tighter until I had to see the Dr. He checked the band and yes it was tight. When I asked why, all he could say was that it happens and there's no real reason why.
I will continue with the same of amount of portions until after Ramadan this year, and then I'll go back for a fill in late September.