I changed a dirty filter on 7/11 and then today (7/14) I happened to take a look at it and saw that it was getting quite dirty. Why would it get dirty so quickly? I don't ever remember it getting like this. The return vents all look clean. We live in a 3 story townhome and had the unit checked and freon topped off in May. The third floor typically doesn't cool too well so we put one of those booster fans in one of the vents (which doesn't help too much). Could that have anything to do with it?
The filter I put in on 7/11 was a different brand. It did look a little thicker compared to the ones I normally buy (Natural Aire). However, the filter I put in on 6/11 and replaced on 7/11 was my normal Natura Aire brand and that was unusually dirty for one month. I change them monthly and they don't normally get like that.