I think it is always better to be honest with your doctor, even if it's hard. Normally a doctor cannot disclose your medical history to anyone else without your consent, although maybe you had to waive that right when becoming a surrogate. Since it was so recent it is likely to show up on tests for a while, so they may discover it as Kricket said. It can take a couple months before your body gets back to normal, especially since you were so far along, and that will show up in the labs. Your labs will show up as if you were pregnant until hcg (pregnancy hormone) goes down <5. It is not unusual for this to take 6-8 weeks after a m/c or termination.
I would highly recommend it because something may be discovered when you are having your inital sonograms.