Spotting at 7dpo and then get period 7 days after??

I need help! Does anyone experienced like me? I had a positive opk and did baby dance throughout my fertile week. However, on 7dpo I noticed some spotting after sex. Spotting continues on and off for 4 days and stopped. I thought that was implantation bleeding so did a pregnancy test but it was negative. 3 days after I'm now bleeding again like pms. Why am I spotting in between my period?