After going through your history, the first doubt that came to my mind is whether you are having ectopic
pregnancy. Very little spotting followed by some bleeding and
severe pain in abdomen is suggesting ectopic pregnancy. The only point against it is negative pregnancy test. However, you have performed this test on 4th day of missed period, on which the pregnancy test may show negative. Now that more than 7 days have passed, please repeat the pregnancy test. If it is week positive; i.e. second line fainter, then rush to the obstetrician to rule out or manage the ectopic pregnancy as the case may be.
Ectopic pregnancy is a condition wherein the embryo gets implanted outside the
uterine cavity - more frequently tube. Since the tube cannot grow as the uterus can, it ruptures causing severe bleeding.
The other possibility is that you have
blighted ovum wherein the
gestational sac is empty and it is a nonviable pregnancy which is aborting.
May be otherwise your embryo has become nonviable and you have landed in a
missed abortion, which needs the abortion to be completed after thorough blood check up for disseminated intravascular
coagulopathy (DIC).
Whatever may be the condition, please do not waste time, go to the obstetrician and get the diagnosis confirmed and prompt management.