always feel like need to move bowels

hi all new member here i am having a problem which my doctor disagrees with me up untill three months ago i was having normal bowel movements even taking pain meds for 3 years(i am on fentanyl patches)one morning i had a bowel movement which was a little tough to move(strained)ever since that movement i have had the urge to go to the bathroom and make a movement and still do to this day it never goes away even when i go to the bathroom.my dr tells me im severly constipated and gave me lactilose to drink but now when i do move my bowels its like little snakes or worms(really thinand flat looking) i had a colonoscopy done and the colon dr never said anything to me about constipation and said i wont need another till im 50.so my dr told me to drink more lactilose which i did to the point i pooped out clear water.i told my dr what happened and he still says i am constipated but now im having lots of discomfort and sometimes pain in my stomache.the pain gets worse when i eat or when my bladder fills.it feels like something is pressing up against my bowels inside but he wont listen to me he wants me to eat yogart cause he believes the fentanyl has ruined my digestive system and my bowels.has anybody had anything close to what i have discribed