What Causes Delay In Periods?

hi, i am 18 years old and within the past 2 weeks i have been experiencing fatigue, 8 lbs of weight gain, memory loss over simple daily tasks and routines, my period this month is delayed (pregnancy is 100% ruled out), my throat feels tight,clogged ears, and i feel like i have a big strain on the right side of my neck/shoulder. help?!