I'm having pain & cramping in my stomach and

Hi,
The fact that you have irregular periods, pelvic discomfort and nausea indicates that your hormonal levels are not normal. There can be various reasons for the abnormal hormone levels. Majority of these causes must have been ruled out since you have had all the basic examination and investigations. Pathologies like fibroid, endometriosis, ovarian cysts are ruled out
The other cause can be -

• Borderline hypothyroidism - sometimes a mildly deranged or borderline thyroid function test (TFT) can cause similar symptoms
  
• polycystic ovarian syndrome (PCOS) - In this it is not necessary that there are obvious cysts in the ovaries. PCOS is a syndrome with abnormal hormone levels and irregular periods seen in females who are overweight and have mildly high or borderline high blood sugar levels.
• Stress - There might not be medically anything wrong. But due to stress, maybe regarding conception can cause symptoms like yours.

If investigations like TFT, glucose tolerance test etc have been done and are normal, then the most probable reason for your symptoms is stress. If these investigations have not been done, then you need to contact your Ob/gynae to rule out the above possibilities.
Regards,
Dr. Gupta