What Causes Light Vaginal Bleeding While UPT Is Positive?

My last period was jan 23 ,feburary I did test and it was positive ,I bleeded for a day which I use vitamin K and it stop, on 2 march I did a blood test which is also positive but scan can t see anything, I have beeb bleeding since then but it was light with thiny clot. I did a urine test still positive but faint line