I don't remember getting a RhoGam shot after my son was born (he's A+, and I'm B-). I'm now pregnant again and worried that my antibodies may harm my unborn baby.
When two different types of blood mix, as in a delivery, the mother will make antibodies to the baby's blood—seeing it as a foreign invader. Of course, that baby's "outta there," so there's usually no problem. That is, until mom were to get pregnant again. If the developing baby is Rh-positive and mom is Rh-negative as with your previous pregnancy, then mom's antibodies can filter through the placenta and attack the developing baby (if he or she is Rh-positive like the baby before).
To fix this situation, your health care provider will give you a RhoGam, or rho-immunoglobulin shot, after your first delivery. The shot deposits a load of antibodies into the mother, making her body think the whole situation's already covered, so no maternal antibody is manufactured. Fortunately, the RhoGam shot's antibodies are such bulky molecules that they can't filter through to the baby, so mom's body is fooled, and subsequent babies are protected.
As far as whether you received a RhoGam shot—or not—after your first delivery, I would advise you to ask your doctor. Checking on mixed mom/baby blood types and giving a RhoGam shot is a BIG DEAL. It's something your obstetrician knows is vital to ensuring a healthy future pregnancy. In other words, you probably did receive it and just don't remember. During all of the flurry of excitement surrounding the birth of your son, you may not have realized your doctor giving you the shot amongst all the other medications, IVs, and pills that you may have been given.
Now that you're pregnant again, keep in mind that your health care provider does a blood work-up during your initial office visit. That blood work should also include information about whether antibodies are present in your blood. (This blood work will be repeated again at 26-28 weeks.) Talk to your doctor about your concerns, he or she should be able to look at your blood work and clear up any questions that you may be having.