Breastfeeding issues. Milk supply?

Hello,I'm mother of 2 month old baby. She was preterm born in 34th week and kept in incubator. She was given with prenan while in incubator. At starting she was unable to latch properly but after one week she started to latch properly and I was giving breastfeeding. Now suddenly I'm feeling my milk is not sufficient to her. Sometimes milk supply is sufficient ,after feeding she is sleeping..but other time (mostly at night) after breastfeeding she is still feeling hungry and I'm giving formula. Sometimes while I'm nursing one side,the other side milk is leaking and sometimes it becomes so dry. She got gas problem and doctor told dat is because of mixed feeding of mothers milk and formula milk and doctor suggested to give mothers milk as much as possible,but I'm unable to see my little one getting strained so much while passing gas or stools. Now my concern is whether I have sufficient milk supply and what to do for increasing milk supply?