Go to your local pharmacy and get a bottle of Magnesium-Citrate (comes in different flavors: cherry, grape). Drink it down all at once (you can add ice as it tastes like sh*t), drink tons of water post ingestion, and be ready to pooh your guts out. Be very near a toilet about 2-3 hours after taking it.
Completely safe for your kidneys. I make anywhere from 10-20 stones a month, and am constantly taking pain meds and getting constipated, so this is the best advice I can give. I'd do this sooner rather than later. You FOR SURE don't want an 'impacted bowel'. Never had one myself, but a good friend of mine on this site HAS, and she say's they're brutal.
Good luck to you.