What Causes Light Menstrual Bleeding While Trying To Conceive?

Hello there just wondering something so here it goes my husband and i have been trying to get pregnant since feb with no luck i have a two year old at the moment but my period last month was four days late and when it finaly came it started how it normally does heave then went very light pink to the point it was not really there lasted four days and stoped i have had sore boobs headaches feeling sick i had cramps when i thought i was on my period last month but not like normal period cramps i tooj four pregnacy tests and they were all negitive im due for my period on monday i can normaly tell when its on its way but dont feel it what the hell is all this mean