When babies are around 3 weeks old (21 days) they go through a growing period. They need more milk, and it is normal to have a baby nurse almost constantly during this time! It sounds like you are doing a great job of nursing n demand. Thats exactly what you need to be doing! When your baby nurses constantly, it tells your breast to increase the amount of milk it makes. Your baby will go through many periods like this, but the first one can really make it feel like you don't have enough milk.
Does this seem like it might be correct for your situation?