According to the symptoms you describe (spotting and abdominal cramps) you might be experiencing a miscarriage. You need expert help and you must see a gynecologist immediately!
If this is your first pregnancy, and you have never had previous miscarriages or abortions and have never received Rh blood then this possible miscarriage is probably not caused by any Rh -incompatibility (blood type) between you and your baby. But to confirm Rh-incompatibility as a cause for miscarriage, anti-Rh-antibodies must first be detected in you and your fetus. Rh-incompatibility between a mother and her fetus occurs if the mother is Rh- and the fetus is Rh . If the mother is Rh- and the father is Rh there are 50-100% chances that the baby will be Rh . A Rh fetus stimulates the mother’s immune system to produce anti-Rh-antibodies against the baby’s erythrocytes. This process is called "immunization".
Anti-Rh-antibodies enter into the fetus’ blood and destroy its erythrocytes (hemolisis) and then cause hemolytic anemia and further heart failure. Bilirubin (a degradation product from the erythrocytes) then precipitates in the brain and damages it. The fetus either dies in its mother’s uterus or is delivered with damage whose severity depends upon the degree of hemolytic anemia.
A mother’s stimulation (immunization) usually occurs at the end of pregnancy or during the delivery, except if the mother hasn’t been already immunized from any previous deliveries, miscarriages or abortions. If the mother hasn’t been previously immunized (has no anti-Rh-antibodies in her blood) the current pregnancy will immunize the mother and the baby will be delivered with no problems. During her NEXT pregnancy, however, if the fetus is Rh , the mother’s blood will contain enough antibodies and enough time to destroy the fetus. To prevent immunization and damage to the fetus of the next pregnancy, every Rh- mother must be given a shot containing anti-Rh-antibodies soon after delivering (no longer than 72 hours) a Rh- baby or if the baby’s Rh-blood type is unknown. Therefore, the shot you’ve mentioned it doesn't seem likely that would help you to keep this pregnancy. If you do lose this baby (miscarry) you can request to take the injection shortly thereafter to prevent another miscarriage.