I agree with Tina. HCG level starts to increase after implantation and it will take at least 10 days before a regular home test will detect it in your urine but that depends on the person. Implantation happens 5/10 days after ovulation.
My hcg level on day 7 after transfer (ivf) of just one embryo was up to 35. It is now 414 at day 18. didn't increase as predicted in the beginning, so we are still holding our breath.
Yes, you can have Hcg in your system. In fact, even men have it :) Less than 5 is considered not pg. Everyone is different as to what the level would be the day after implant. Also, are you talking blood or urine. The day after implant is not likely to show on a hpt. It can take some time to build up enough to show in urine.