So on saturday or sunday my girlfriend's middleback-lowerback was a bit sore (her muscle?) and it went away on tuesday and came back today.

Its on her right side on her back where her ribs end and her torso begins. She said it hurts to walk as it stretches the muscle and the pain extends to her waist. She said its only her right side.

As always i've been googling and they say it could be a kidney problem?

her period is supposed to come saturday, could it just be pms?