Is It Safe To Take Miralax For Chronic Constipation?

Having a been diagnosed with a severe case of constipation and having taken Miralax for 3 days and then having a substantial bowel movement over the course of a couple of hours or so, I find I am still having discomfort in my lower abdomen or intestinal area with some cramping and some churning of that area. Any idea if I need to continue to use the miralax or something else such as Dulcolax in case I did not get cleaned out completely??