Worried that my milk supply isn’t enough for baby

I’m exclusively breastfeeding on demand. My baby is 18 days today and has been feeding every 3 hours and at night she goes 5 hours without feeding. I’ve noticed that these last two days all she does is cluster feed the whole day and last night she woke up every 2 hours. I’m worried that she might be hungry since she’s constantly feeding and I’m not really sure how much milk she’s getting. I just gave her a bottle and pumped for 20 minutes and only got 1.6 oz from both breasts. Is that ok? Now she’s been sleeping for 3 hours and I’m pretty sure it’s because I gave her a bottle and she got all she needed at once. Her dr said that her stomach’s capacity at this age is 2 oz but I’m just scared I might not be producing enough for her.