What Causes Late Periods And Light Vaginal Spotting?

hello there im a 24 year old woman regular periods every 28 to 29 days always very heavy and painful to very light and gone. My last proper period was may 31st i was due on the 28th june as that was my lil bros birthday nothing came and also normally before im due on i get pains etc and know im going to come on i didnt get any of this was very differnent for me well on the fri the 2nd july i started to spot on and off rite till the sun which is nothing like me ive done preg tests all say not preg ive got sore boobs bad back a horrible test in my mouth im very moody and soooo tired its unreal i already have two children but this is confusing me ive never been late unless ive been preggo and ive never spotted what could this be please?