Spotting between af and ovulation??

Mummyto2lovelyboys💙👬
So I normally have af for 5 days, yet this month it lasted 7 days. On cd 8 I had alot of brown cm. On cycle days 11 and 13 I have had watery blood spotting. What could cause this? I know it's not ovulation bleeding as my opks have been negative so far. Can anyone shed light on this or has anyone experienced similar? Tia xx