first not need to worry .
this is very much sure that you are not pregnant . as blood test
for HCG is also negative .
u should consult to gynec.
go for whole abdomen USG.
cause of delaying in menstruation
1.some time it is normal physiology in delaying perod
is important cause of delay period . to avoid this you should do some meditation
3.usg will help you to find out any pathology like PCOD , cyst.