Thanks for writing to DoctorSpring.com with your health concern.
With 3 negative pregnancy tests, it is almost 100 % sure that you are not pregnant, unless you have a history of irregular cycles, which you have mentioned you do not have.
There are various factors which might delay your periods - anxiety, stress, tensions on the personal or professional front, weight gain, changes in sleep or eating patterns, ingestion of medications that affect the hormonal axis, change in climate and altitude - these are just some of them.
If these are reasonably ruled out , then do not worry, once in a while, a period might be delayed innocuously.
If the remaining cycles are regular, this odd one out cycle might happen once or twice in a lifetime. If you do not menstruate within the coming week, then you would need to see a gynaecologist, to rule out other serious conditions that can cause menstrual delay - PCOS ( polycystic ovarian syndrome ), hypothyroidism, endometrial hyperplasia etc.
For this you would need -
1. Internal check up - per vaginum and per speculum examination.
2. Pelvic ultrasound scan
3. Blood tests - FSH, LH, TSH, Prolactin, OGTT, lipid profile, DHEAS, free testosterone, fasting serum insulin.
This workup would reveal a pathology if it exists.
As you are having some kind of pains and backache etc, it is likely you would have your period within the coming week.
Wait it out for a week, and then have the outlined workup done.
Please feel free to ask for further clarifications.