Why do I have a sharp pain at the bottom of my left ribs? It hurts once every month and it is not my ribs that hurt, it’s at the bottom.

It has been happening for 2 yrs. All of the sudden it just hits me and it feels like someone sticking a knife in me and twisting it. Sometimes it hurts more than other times. One time I fell down cause it hurt so bad. By the way I am 14 in 1 months I will be 15 if that helps