Black Stools, Stroke Patient, Taking Bisoprolol And Gabapentin. Immediate Medical Attention?

My mom is a stroke patient, not able to vocalize, bed bound and on NG tube feeding. She had constipation issue and been given lactulose (about 20ml at night and another 5-10 ml in the morning) as required and seems to help. Lately, about more than a week the lactulose (20ml night and 15-20 ml day) didn t seems to help much she pass out very little each time and if she managed the stools are mostly black and very thick. She s on 5 cans of Glucerna with 200ml water in each feeding. Her medication include 100mg Aspirin, 2.5mg Bisoprolol Concor in the morning, 10mg Simvastatin, 300mg Gabapentin at night. I am not sure if there are underling issue and if I should call an ambulance and sent her to the doctor? please advice what I should look out for and If there is an immediate need for medical attention. Thanks.