Drinking too little?

My baby girl is 3 months old in 2days time. She is drinking less than 90ml per feed(nan optipro), about 70 to 80ml every 2 to 2.5 hours and sometimes 60ml. At night, she can sometimes sleep thru or she wake up to drink once but also can only finish 60ml. Is she drinking too little? How can I make her drink more?