Having just had such an issue, I think there's a high probability that in the OP's case it wasn't a muscle issue, but rather a strain in the iliolumbar or iliosacral joints. Or so said my doctor. A better fix than changing one's bike position is just riding Z1 for say 30' a day for a few days to encourage healing. Fixed me right up, pain gone. In my case, I think I did it at the gym. Now back at the gym, same exercises, no pain after or on the bike. Wonderful to have the ability to self-repair. Wish my car did that.