FTM, please help me 😩

Amber • 20 years old, mumma to my beautiful boy Oliver and expecting my second in Jan 2018 👶🏼
I'm a FTM and my Bub is 18 days old. He's been feeding well up until the last week. I just haven't been filling up enough to satisfy him and I don't know why! In the morning I'm usually pretty full, feed him, put him down, and when I would normally have enough left to express a bottle to put in the fridge, i have nothing! He usually sleeps for 3 hours, and even then by the time he wakes up I don't feel like I've filled up much and after feeding him for as long as possible he will cry and not settle. I thought maybe he was full and just gassy or something but when I rub his cheek he still turns his head as if he's still hungry! His cries just hurt so much because I've tried everything, I drink lots of water, have been eating reasonably well, nap throughout the day!  Im starting to think that I should get some formula just as a top up for the feeds where he still isn't satisfied. Has anyone else needed to do this and been successful with it? I don't won't to stop breastfeeding all together but I don't want my baby to go hungry 😩