Wa'alaikum assalam sis! Honestly, some back pain around that area is pretty common during pregnancy, but if it's very sakit macam tu, better check with your doctor or gynae just to be safe, could be ligament pain or something else they should know about, especially since you mentioned it started after the injection. A few of us mums experienced different reactions to the vaccines during pregnancy, so it's good to mention the timing to your doctor so they can check properly. Hope you feel better soon!