What lower back exercise help back pain?

I have F cup icles and I play roller derby. I find that if I skate for more than 7 or 8 minutes I get really bad back pain and once it comes, even if I stretch out best I can as soon I go back onto the track within a minute or two its hurting again. I am a big girl, and am losing weight to become a better athelete. But whilst I still got these I need some kind of stretch advise to help? Any reccomendations?