What Causes Spotting With No Menstruation After Intercourse?

my name is remya , age - 23, weight- 50 kg, height- 150cm i had d and c jan 23rd 2011, my dr advised me to take yasmin tab , feb mar , apr , i took this tab , may 1 st my period occur , then 7 th again i started the next packet ,7th to 9th i took tab , that time early mrn i felt nausea , vomiting , 10 th to 13th didnot took the tab , on 14 th i took tab after sex , on 15 th& 16 th spotting occur ,i do the pregnancy test on june 8 th , it was negative , still i have no period