period stopped

Chelsey
hello all! so i have not had a period in over three months. no i am not pregnant. sadly but since ttc a year ago my period has become irregular when it used to be regular.. has this happened to anyone else?! advice??
also i am an avid runner so I believe I most likely have to cut back on that even though i have always been but anything i search all signs point to exessive exercise/lack of nutrients. again thoughts? advice? 
helppppp