Hi Qin,
I hv the same problem after starting taking multivitamin n calcium. Two days of pushing in the toilet. But I solve my problem by adding more fibers into my diet. Fiber to be taken in-between meals not together. For example:
Morning empty stomach: one big glass of warm water. A plate of fruits (apple, peach, kiwi, dragon fruit,...), a glass of juice or milk with 2 teaspoon of pylium husk (Chinese physian recommend). Rest for 30mins, eat breakfast.
11am: a plate of fruits: Prunes, strawberries (higher fiber), salads, mini sweet tomatoes.
12noon: lunch (with more vegetable)
3pm: a plate of fruits n salad (refer to 11am)
7pm: dinner with more vegetables.
I m back to normal now. Very happy but my fridge is full of fruits n veggy n salads. Good luck.