For a past few days, my stool is coming out like small pebbles, nut-shaped, dark in colour initially, then its normal, pain in rectum before defecation?

Small stool.: Your description of your stool shape of small pebbles and pain before defecation is likely secondary to lack of fiber and hydration. The colon will absorb a large of amount of fluid by the time you have bowel movement (only about 100cc-200cc left inside the stool). Suggest to drink adequate amount of water, high fiber diet, and take fiber supplement (citrucel 1 tablespoon in 8 oz of water, daily.