What Could Cause Irregular Periods?

The first day of my last menstrual period was 15th of february...but my period was due around 20-23rd of feb....it wasn't a normal period I didn't bleed much ..just lasted for not even 2 days..(normally its lasts for 3-4 days). And I missed my period in March ....home pregnancy test turned to be positive...I went to my GP he told me I can't get the appointment with my GP until the 12 week....A friend of mine just told me ..it may be implantation bleeding what i got in feb.....now I am confused...when did i actually conceive? was it in feb?or march?