Hello. The system says that you have requested a different expert.
The greatest concern that could be causing significant pain in the lower abdomen and pelvis and thigh would be an infection of the female genital tract. It is impossible to determine whether there is an infection based on symptoms without performing an exam or other tests, which is why it would be better for her to be seen.
If this is milder pain, similar to what she gets during a period, that would not be as concerning, and she may be having uterine cramping, even though she is not having bleeding indicative of a period. So, if the pain is mild and similar to cramping that she gets with a period, it would be reasonable to wait and see if the pain eases spontaneously. It would be fine for her to use over the counter medicines for pain, such as naproxen or acetaminophen.
However, the other expert is correct that a pregnancy test is far more accurate at determining whether a woman is pregnant than subjective symptoms, assuming that it was done correctly. So, if the test is negative, that would indicate that the absence of a period is related to the birth control pill.
If I can provide any additional information, please let me know.