Negative blood question

So I'm O- and have given birth twice and gotten the rhogam shot everytime. I'm 12 weeks with my 3rd and just got results from my blood test and the nurse said there are faint amounts of postive antibodies in my blood. They want me to do another blood test to try to figure out what it is but she really had to no other info for me. Does anyone have any experience with this or knowledge of what this means for me and my baby?