Spotting!?

I’ve been spotting for 3 days on and off. It ranges from light pink to dark brown. It’s never been heavy enough for a tampon, but it has come out in a glob once a day (still only a quarter size on a pad)... sometimes there is NOTHING when I wipe and/or on the pad... I started spotting 6 days before my period (which is 28 day cycle like clockwork so this is SO weird so I don’t think it is my period).. 13 dpo and a negative test yesterday. I’m just so confused! Anyone had a similar experience!?