If you can, I highly recommend speaking to a lactation consultant. It’s pretty rare for your supply to be insufficient unless you’re on medication which affects it, so there might be another reason she’s getting fussy. I will say that being dehydrated definitely affects my milk though.
I posted about this only a few days ago and got some good advice and since then I’ve been drinking coconut water, eating overnight oats with honey, chia seeds and flaxseed every morning and drinking at least 3L and I’ve 100% noticed a difference, I feel fuller between feedings and having to pump in the morning x
https://www.emmapickettbreastfeedingsupport.com/blog/low-milk-supply-101
Ensure your latch is correct. You're eating enough and LOADS of water. Then keep putting baby to breast. Swap back and forth during feed is good too. Drinking enough is vital for milk supply.
Oats for breakfast, postnatal vitamins, mothers milk tea and lots of water throughout the day. Like double of the normal allowance, I probably drink 4 litres sometimes more because I'm so thirsty. Read the link Melissa sent, you likely don't have a problem with your supply. If you really want to work on increasing though, pump after every feed for two days or power pump
Fennel tea is good, Twinings sell it x
Loads of Water, coconut milk and porridge oats really worked for me
Oats are good for milk supply