My friend had an abortion and I remember taking her to her follow up appointment a month later. She said they did a pregnancy test. If you have a positive pregnancy test and have not had sex after the abortion I would say maybe they didn't get everything out? I don't think you should be getting a positive test from the abortion though.
Usually after an abortion/miscarriage/D&C the pregnancy hormone levels go back to 0 within 6 weeks. I would say this may possibly be a new pregnancy. If it is not a new pregnancy it could be that your body still has elevated levels of the pregnancy hormone.
Either way you should get an ultrasound when you think you're about 8 weeks along to see the baby and heartbeat.
did you have any sex after abortion?