G.,
The same thing happened to me when my son started on solids.
When babies start solids, they take less milk.
Babies drink more from a bottle than when they breastfeed. It is easier to drink from a bottle. It's less work than drinking from you! This is one reason formula fed babies tend to be heavier than breastfed ones.
A few things you can do
1) make sure to breastfeed immediately before you to work and when you get home.
2) If you will be home soon, have the caregiver feed the baby one ounce at a time and wait a bit.
3) Replace the valves on your pump and make sure the parts are all clean and well put together.
4) Take some "more milk plus" or fennugreek. It can increase milk supply.
5) Drink lots of water and get as much sleep as you can!
6) You can mix formula and breast milk.
7) Call La Letche league. They are super helpful (and free!). They may have more suggestions. The yahoo group "pumpmoms" is also super helpful.
Babies have growth spurts, and time they gain less weight. If you baby looks good and is gaining weight, don't worry about the lower milk production.