Missed period

A little confused ... My periods come faithfully on the 3rd of every month, for, four to five days... but in March it was wired ii had a brownish discharge for a couple of days then on the 6th of march i had a very light pinkish colored period only lasted 2 days took a Prenancy test came out Neg. Now its April my period still has'nt came i've taking another PT. Its still coming out Neg. What should i do its stressing me totaly out im 25 with no kids.