What Causes Stoppage In Periods?

hi Dr, I once had sex with my boyfriend o nthe 18 January 2014 tryin to concive, i did see my period and i check pregnancy test and scan it showed that am not pregnant,well i continued to have sex with my boyfriend on the 15th,21st and 22nd February 2014, well i did pregnancy test yesterday on the 2nd March still it shows that am not pregnant, even my ovulation test yesterdy.now because i havn t see my period we want to start thins fresh and we decided to take yasmin pills and am also taking folic acid pills, it is ok?was think that yasmin pills makes period reggular because once i start them on the first line i have to see my period after seven days so i was thinking to atteck before period,is that a good idea because i want a baby so bad.please hide my ID