hormones messed up or pregnant??

so my cycles for the last two or three years since having my son have been 3 weeks apart approximately. last month me and my SO had sex with no protection on what I thought was the last day of my cycle this happened on December 10. I was spotting this day so i considered it part of my period. After I showered and everything my spotting stopped completely and I didn’t realize it and we had sex. my period is now two days late and tonight 12/29 I just started having pinkish spotting. it’s mixed with CM. usually my period starting is pretty abrupt and I will feel a real flow. The test today was negative. Is it possible to be pregnant and just not test positive until after implantation occurs?