no poo affecting diet?

My LO is 7wks now and 80%BM 20%FM. She hasn't poo for 2 days and her intake seems to be dropping. Her usually is 600-650mL bm+fm. But ytd she only drink 540ml. Usually she will request a feed at night and she doesnt, and in the morning 7am, she only drink half her usual feed. Typically it is 90ml per feed. Is it because she doesnt poo so it affect her intak- wet diaper are normal. Thank you!