This is a common issue. Do not worry it is because baby is used to easy drinking of formula milk. Breast feeding is a more tiring procedure and baby has to first latch properly and suck to start getting the milk.
Next Steps
Only if you start to feed, you will have more milk. So feed more frequently
Health Tips
Please check the positioning and latching of the baby. Aldo see that your nipples are not cracked or inverted which will again be a problem