It is impossible to say if the infarct was the source of the blood that caused the sensitization.
Some women become sensitized without evidence of any placental damage.
It is more likely that it was from it, though. A good pathologist might be able to give some idea as to when the infarct occured, but it may not be very accurate depending on when it occured. There can be symptoms of infarct, but most likely, there is no suggestion that it has occured. Without knowing if in fact there was blood from the infarct, can't guess if it would have been covered. If there was erythroblastosis with this pregnancy, it is more likley that you became sensitized previously, as the first exposure develops antibodies, and subsequent pregnancies are at risk.