Acute constipation. Hard to pass stool. Took miralax (polyethylene glycol) 3x since yesterday, only passed a few pebbles. Abdom and rectal pain. Colon cancer in family. Help?

Constipation. First, constipation by itself is not a specific sign of colon cancer & colon cancer is very unlikely at age 21. Drink more water. Eat much less solid food until the constipation resolves. Can try suppository or enema too. Try stronger laxative like magnesium citrate. Avoid this problem by eating right, avoid junk food & excess alcohol, eat more fruits & vegetables, add metamucil. See your doctor.
If oral laxatives. Are not helping, you might need to resort to an enema, like a fleet enema (sodium bisphosphate). If this doesn't help, you need to see a doctor for mechanical disempaction. Don't wait too long on this one.