Brief Answer:
There are many possibilities.
Detailed Answer:
Hello and welcome to healthcaremagic,
I understand that you had missed your period for about 7 weeks now and home pregnancy tests were negative. 
You have also said that earlier pregnancies were detected late at about 8 weeks. So the chances of pregnancy cannot be entirely ruled out in this case.  A urine pregnancy test done using the first morning sample of urine will give better results. Also it will help to know if your periods are regular or not. 
Irregular periods indicate a delayed ovulation and hence pregnancy tests will come positive later than usual. Please check to see if this is your case. 
A 
missed abortion [ pregnancy occurred but the baby is not alive now] will give rise to pain and negative pregnancy test. 
Other causes of similar symptoms are ovarian cysts that lead to hormonal imbalances and irregular periods with 
abdominal pain. Fibroids may also produce pain but missed periods are rare. 
Endometriosis is a condition where 
menstruation occurs inside the abdomen. It causes pain but again missed periods are rare. Another common cause of delayed periods is stress and hormonal imbalances. 
The best way to know the cause is to have a vaginal examination by a gynecologist and an 
ultrasound to look for any of the possible causes. If it is negative a hormonal profile to look for levels of FSH, LH, TSH and 
prolactin will help. 
Hope I was able to answer your query. 
Feel free to ask any more questions that you may have and I will be happy to answer. 
Thanks and have a good day.  
Dr Madhuri Bagde
Consultant Obstetrician and Gynecologist