A home pregnancy
test taken soon after conception may not get you a positive result as those tests are not very sensitive. Since you did not give the dates of your cycles, it is difficult to say, yet you can repeat the urine test and see and in case of doubt, go for the blood test
followed by a trans-vaginal sonogram. If the pregnancy is confirmed and if you continue spotting, it needs the immediate attention of your doctor as it could either be an implantation
bleeding ( harmless) or a threatened abortion
(harmful). Take care.