Tanong ko lang po (mas ok kung may OB dito, need lang ng advice)

Tanong ko lang po. First time po kasi nangyari sakin to. Usually po 30-35 days po bago dumating yung menstruation ko. My last menstruation is nuong Dec. 28, 2019 and yung Scheduled na period ko for this month is on the 23rd to 27th day of January. So since between 30-35 days dumadating yung menstruation ko, yung Ovulation day ko is nuong 12th of january and may nangyari po smin ng Boyfriend ko on the 13th of january. SO, eto po yung nangyari, on the 19th of january mga around 10pm, nagulat ako ng dumating yung menstruation ko, but its nit the usual menstruation kasi may pagka Pink siya and it lasted for about 2 days, it's not my usual menstruation kasi hindi nya napuno yung napkin ko and nung ika 3rd day is Brown Discharge nalang yung natira. Nagtanong ako sa mga kakilala ko na may idea about family planning and sabi nya, the normal cycle of period is 28 days, so kahit mag minus pa daw ng 2 days prior on the 23rd, pag daw 28 days cycle ako dapat 21 pa darating yung period ko, which mean mas early pa din yung january 19th na period or spotting since it's very unusual and first time nangyari sakin. The question is : am i experiencing Implantation bleeding/spotting/ or mabubuntis po ba ako?? Nagtake ako ng Pregnancy Test last monday, january 27th pero negative naman siya. Please po. I need help and answers. Thank you!