I've had no periods for 3 months and i'm really tired and sleeping all the time not pregnant any ideas?

Amenorrhea: Would guess that this is likely due to hormones, stress, poor nutrition, or change in diet, exercise, or eating habits. Might be time to go and see a physician and have your hormone levels checked. Usually it is felt if this goes on for over 3 months, time to get it evaluated further.

Get checked: You need to be checked, most importantly, for thyroid disease. Decreased energy can be a sign of hypothyroidism, and abnormal thyroid levels can effect your menstrual cycle.