What Causes Delay In Menstruation Other Than Pregnancy?

I am 23 years old, and have one daughter. I just quit taking the pill in April, and I had a period in May. It is September now and I haven't had a period since May. I have seen the doctor three times this past month and I am not Pregnant and have nothing wrong. I've had blood tests done they have checked my hormones, thyriod, and blood sugar. The doctor said that it might be stress but I don't feel stressed. I am a little worried because I have lost 10 pounds in the last month without trying....What else could be wrong?