Hey mamas! I need some input.

My almost 4 month old little girl is pretty regular with one big bowel movement a day every now and then she will skip a day or two. If she skips two I usually give her a little milk of magnesia (which was recommended by our pediatric nurse when she was a newborn and had gone3 days without a BM). She usually has a bm within 12 hrs of getting the medicine. She has been a little constipated I think for about 5 days. She's skipped a day here an there but when she would go it was just a small amount and much thicker than usual. And yesterday she screamed for 30 minutes and nothing I could do would help her. Finally I noticed that she was grunting before starting a new scream. So I gave her some Milk of magnesia. She had had a small bm that morning and a few hours after the medicine she had what I would call a normal bm. Now this morning she had a big bm that was very loose but not watery. What worries me is that it had dark green mucus all in it! I've never seen anything like that in her diapers. Of course Google says everything from its a virus to its a teething symptom. Has anyone else had this issue? Should I be concerned?