help! milk supply question

My baby is 17 days old and out of nowhere yesterday my milk supply changed. I noticed him getting very fussy while breastfeeding, practically screaming at my breast. When I pumped I realized my right breast was barely giving me milk and while my left is better it's giving me less than it did the day before. I haven't changed anything and I'm so scared I won't get more milk. With my first baby (4 years ago) I always had a lot of milk until I got mastitis at 3 months. I'm going to pump every 2-3 hours and drink A LOT of water. Anything else I can do? I really wanted to breastfeed longer than this.