Hi and welcome to HCM!
Thanks for posting your query here.
The result of the pregnancy
test depends on the timing of the test done. If the test was done during menstrual period the blood mixed in the urine may not give a true result. A morning sample will be more accurate. If you are on any birth control it could be withdrawal bleeding.
Or it can be a change in your menstrual cycle. The symptoms you mentioned could be related to premenstrual syndrome
symptoms. If still the doubt persists a sonogram will be helpful to you.