Symptoms from Dieting?

Hey, I've been on a diet steadily for about 5 weeks now (started beginning of December), and I'm on a plan of only 1200cals a day or less. Everything has been going smoothly, I've been loosing weight, but ever since I've been getting weird symptoms, and I'm wondering if it's because of the diet or something else? I'm like 3 weeks late on my period (I might skip it! never happened to me before), I've been developing white phlem in my throat for the past week, I've been getting more sleep and I'm still tired all day, and I've just recently getting sores in my mouth (1 is a sore, and the other is a blood blister i think.. it popped and blood came out) so I'm kind of scared should I stay on the diet? will these symptoms p or should I worry? I'm 18 btw