What sort of test was it? Some are more sensitive than others so if you are 3 days before your period you could easily be pregnant and a test which is only 25mui wouldn’t have picked it up.
I read recently that implantation is most common 8-10dpo, so assuming you have a normal 14 day luteal phase and your period is due in 3 days, you would be 11dpo and you could easily have implanted yesterday or day before and not have enough HCG yet. Not out until your period arrives
Ttc is a tough old time. Best of luck x