Low vitamin d level (19), high phosphorus level (4.5), high pth level (70). what could be the cause of this? serum calcium level is normal.

Vitamin D deficient: Deficiency of vitamin D will elevated the parathyroid hormone level. The phosphorus elevation is minimal. You need to address why you are vitamin D defricient, consider supplementing more than you are doing already, or if you're on a good oral dose, consider whether your "irritable bowel" might be lipid malabsorption. Get yourself a healthy amount of sunlight. Best wishes.

Treat low vit D: I would get the low vitamin D treated and then recheck the labs. The 25OH vit D should be above 30. This will likely correct any of the other mild reported lab abnormalities.