TMI!!!! BM question

So I'm currently 17 weeks pregnant and have had very irregular BM my whole life but since I've been pregnant they've gotten much worse. I take mirilax to regulate but it doesn't do anything. I'm lucky if I can even go once in 3 days. But when I do go, my stool has a thick white coating covering it. Anyone else had this and know what it may be? It's also very painful to have a BM. I'm worried about why it looks weird. Thanks in advance