I have severe constipation with constant nausea

i have severe costipation and it has been going on for four months. i went to the doctor a he gave me this medicine called miralax to help me make a bowel movement . i was on it for three weeks, and i wanted to stop so that i could try to make a bowel movemtn on my own. i got constipated again so i bought a store brand of miralax and i took it, i drank water and exercised and i am still constipated, i took some last night and early today and still have not been able to make a normal bowel movement. I have changed my diet to try to help it and it worked up until recently. it got to a point before i went to the doctor that i was puking and constantly nauseous and dropped 25 pounds. please tell me what i need to do, or what i have so i can fix this because it is making me miserable.