Technically, it IS possible to be pregnant, but highly doubtful. That being said, it is possible. In researching, the soonest I found was four weeks post-partum that someone became pregnant. Since your menses probably will not start for at least a couple of months, the only way to know for sure would be a pregnancy test by blood work.
That being said, usually, doctors want you to wait about six weeks after delivery, to give your body and sex organs a chance to heal.
In any case, if you are not wanting to be pregnant right away, you do need to use protection/contraceptive. If you are not breastfeeding, you can speak with your physician about going on oral birth control, or depoProvera, or other method or device.
I hope this answered your question. Please let me know if you have any further questions or concerns.
Best wishes to you and the new baby!