Suggest Treatment For Severe Constipation And Blood In Stools While Having IBS

I found out I have IBS and lactose intolerance in June of this year. Since then I have changed to all lactose free dairy and have done very well. But I am also experiencing a lot more constipation. Currently I am going through a bad episode of constipation which causes my hemerhoids to be severe. Last night I noticed blood in the toilet around black stool and this afternoon I have noticed string like things hanging off some of the stools. I took a closer look and it was more fiber but I am starting to get weirded out! Is it possible I have a parasite or maybe a bacteria? Should I go to the doctor?