Hey there,
The cramp you’re feeling is normal… I recently finished teacher training, and Bikram explained that it is occasionally normal for students to feel a cramp there. He suggested coming out of the pose when the cramp gets too intense & to do a slight Camel Pose to release of the cramp. I have felt the same cramp… but more towards my chest. If it’s not too intense, I am able to finish the pose and continue with the rest of class without doing the back bend to release the cramp. You are doing the pose correctly by keeping your stomach nice and tight… any forward bend you do, you want to keep a tight stomach to support your back.
I hope this helps.
Keep us posted,
Tarah