What Causes A Missed Period With Back Ache?

i havent had aperiod since august.. actually had 2, ashort little one follwed by another one in august, and then nothing since. i have symptoms of a period. fatigue, back ache, even cramps and irritable but still nothing. Im 48 and used to be perfectly regular. now i never know when its going to happen. i was told i have some fibroids and my uterus was a little enlarged but nothing to be alarmed about.