The quantity of brown liquid is the normal inner lining of the uterus expelled during the period. It is darker and larger in quantity because you missed several periods and the lining has grown thicker. Now, all the lining is expelled together with a greater quantity of blood (brown color). This is considered quite normal when missing several periods in a row. You might want to visit your gynecologist if the same happens after one month during the next period and check for any hormonal imbalance.