If you are exclusively bf it's likely that your supply has just adjusted to your babies needs rather than slumped so you don't feel so engorged, there is still enough for baby though. If you are worried you can try the following
1. Eat well and drink plenty of fluids
2. Take fenugreek
3. Eat oats
4. Express after each feed
Also, the more baby feeds the more milk you will make. Spend a day or 2 snuggled up with lo having lots of skin to skin and let him suckle as much as he wants.