Welcome to Health care Magic..
I have gone through your query. Based on the information provided by you following are the possibilities.
1. Genital Infection- abdominal cramps , frequent urination
, nausea and missed period you have been experiencing supports this diagnosis. You have not mentioned anything about white discharge which if present and found to be foul smelling would definitely go in favor of a genital infection.
causes- An imbalance in the hormones estrogen and progesterone
can cause abnormal periods. Even decreased thyroid hormone levels also causes amenorrhoea.
3. Pseudocyesis- It is a psychological condition where you experience all the symptoms of pregnancy
like amenorrhoea, nausea, abdominal cramps etc and the person thinks she is pregnant. Stress is the major factor that is responsible for this.
Based on the above possibilities i would like to advice you to undergo hormone analysis ( to rule out hormonal causes), ultrasound
( to rule out genital infection or any structural defects or any ectopy). Please consult a gynaecologist and Based on the test results treatment can be initiated.
Hope you got the solution.
Wish you a very good health