now this month your period is delayed.
you have done pregnancy
test and it is negative.
you have not mentioned about medication.
are you on oral pills?
or have taken emergency pill?
brown coloured is menstruation
is not a problem always.
collected old blood sometimes get darkened and become brown.
this may also cause cramps.
probably you are having this normal variation.
so need not to worry.
brown colour repeatedly for more months may be abnormal.
for cramps you can take paracetamol750/1000mg.
if it is more you can consult physician.