3/4 days late negative and no period

My huband and i are trying for a baby and havent the best of luck yet. My periods are normally very accurate however this time round im 3 nearly 4 days late. Last couple of days ive been experiencing cramping, nausea, lower back pain and not feeling 100% . I have taken a pregnancy this evening and unfortunately its negative which is very disappointing. Could there be another reason for this ? Or should i be taking pregnancy tests at another time ?