When did your LAST period start? When did you have sex with the ex? We need that information.
Assuming you were to start on January 25...
Having sex a few days before January 16 you'd be likely near peak fertility.
Having sex on the 21st would likely be too late, you'd be approaching your lowest fertility. Around the 23-26th you'd still also be unlikely to conceive.
It looks like the ex is the father - if you are pregnant.
Pregnancy tests are really only accurate when they indicate a pregnancy. Testing early, false negatives are common. You were likely pregnant when you tested. For some women a home test NEVER works.
Try again. You MUST use your first morning urine when you test. Use a name brand test as well. If still nothing, see your doctor.