A.,
It is summer and I hope you've adjusted your fluid intake to meet the demands of the heat. Also, you may want to consider what the other responder said about hormonal medications such as the pill, which can drastically cut back your milk supply. Though nothing much may have changed in your diet, have you been eating foods with sage or other things that curtail milk supply? If your son is still doing well, he may be getting the same amount and this would point to something at work.
Sometimes, women become so complacent about pumping at work that they don't really get as much of a let down and the pumping is not as effective. That doesn't mean your son's nursing is not effective, though. You may want to nurse him more often at home and even pump a little more at work. You could consider looking at a picture of him while you pump or even listen to a recording of him during that time.
Some moms end up only nursing when they're home and even nursing all night, but I suspect, with a little work, and investigation, you can build your milk supply right back up.
D. S (mom to 5, granny to 1, CLC)