Why cant I poo?

Yest was the 3rd day that i couldnt poo. Mummy gave me a suppository to help me to poo. The pharmacist advised that this particular suppository was safe for children and that it could be given once every 2-3 weeks if i cant poo for more than 3 days. Mummy tried making me sit on the potty to poo, when that did not work she asked me to stand and try to poo. That didnt work too :(