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?

Ask OB or get record. The administration of Rhogam would be recorded within the ob's office record or the hospital record at the delivering hospital. Contact either & ask specifically for a review of that activity. Hospital records would have it on a medication administration form, a dr would have it listed during the visits involved.You can be tested for this kind of antibody formation.
Checked for early. It is possible that you did receive it and just don't remember with all that goes on after giving birth. They do check for this very problem with the first set of prenatal blood tests.