My premature baby has trouble pooping

she was born at 25 weeks she is now 10 weeks old at first she was on breast milk but since my milk has decrease and it's not enough they start it giving her formula but I notice she's been having trouble pooping for the next 6 days and I talk to her nurse and they are just telling they would watching that since she is having a very small amount of poop like the size of my finger tip and know she's trying but no luck I hate to see her suffer and by the way she is still in the icu.. and do y'all now any pills that I can take to increase my milk.. i have been taken milk Thistle but I don't see that's it's working any advises.. thanks