I am 14 years old. I first got my period when I...

: Managment
When we talk about normal menstural cycle it come every 21-40 weeks in normal girls and it si regular and lasts 2-7 days , But in your case when from last menstural cycle about 50 days it is long period and you should do examination to detect cause , In some cases if there is abnormality or problem in ovulation occured menstural cycle will delay which cause due to lack of progesterone which responsible of bleeding and increase of estrogen which is buildup and withour  progestrone will not occur bleeding , and this cause due to immature eggs or problem with ovulation , in your case we can treat by oral contaceptive pills to regulate your cycles and advise you to go to obstetric and gynecologist doctor to follow your case 
: Diagnosis
 Advise you to go to obstetric and gynecologist doctor to take detailed history , do good examination and some investigations as hormones level and Ultrasound for uterus and follow your case