Spotting

Charley
So girls I am cd 30 today 26/05/16 and on the 24th I was due af it didn't come so that evening when I wiped I had pink blood very very light with brown there to ...
So I thought that was the beginning of AF anyway 25th I didn't want to use tampax so put a sanitary towel on and I had some brown blood and tint but pink but I have really regular periods I am either normally a day early or a day late never any longer ? 
I have done test all negative no dint lines and today u now have back pain in be middle of my back can anybody help please or has anybody had this ???  Xx