I have pain in my left side of my back. I havnt got a kidney infection as been tested but I have big breast could that be the cause of the pain?

Yes. Unilateral pain upper back in twenties female with large breasts might reflect chronic rhomboid major muscle spasm. Each breast can weigh 6 pounds. I have found many large breasted women have anterior tilt to their shoulders, which can stress the rhomboids. Increased tone (yoga) of upper back muscles is advised; along with more of a military posture of the shoulders.