Stupid Mom

My son got mad and told me i was stupid. i wasn't sure i herd correctly but he repeated it again. I stopped everything i was doing and let him be. he tried talking to me and i sent him away with his dad. after a while he comes to me and asks if im mad at him. i tell him no im sad with you. he goes why? because i call you stupid? i told him yes its a very mean word to say to anyone especially mommy. he apologized said he promises he won't do it again. i just don't know how to feel about this. Hes always a loving, caring boy with me. im due Friday with his first sibling and im wondering if hes acting out because of that.