Im not sure that 1 week after conception would give a positive result of pregnancy but if you had got a positive and then had period and then all results were negative then that would probably mean youd had a loss.
there are no figures as to how many losses occur at less than 4 weeks as many women dont even know they are pregnant at that stage, but it is thought to be a high percentage. The foetus is reabsorbed into the body and many women never know that they were pregnant and lost.
positive results can be as early as the first day of your missed period, if you did a test then it is a loss, if it is only a cuple of days since the loss a test willstill be positive as it takes a few days for the hormones to come down.