Unsure whats going on


I have irregular periods and I dont track my cycle. Yesterday I started having light pink-brownish spotting enough to get on my undies and visiblr when wiping but not enough to wear a tampon or pad. Then I realized I’m not due for AF till next week. My husband and I have been having A LOT of unprotected sex lately and it just clicked that it could be implantation bleeding but honestly im not really sure. I have no other symptoms except lower back aches here and there. When is rhe earliest I could test? Took a test today but came out (-).