Why does my pit bull bite me?

6 months old and I've had her for 2 months. I let her sniff around for a while but at a certain point I want to walk at a steady pace. WHen we get to this point she starts jumping up and biting me and my clothing and will go in circles around me doing this. I have to push her down gently to make her sit, and she will stop biting but when we resume walkin she just starts it again. I've tried telling her no, tried ignoring her, tried gently holding her mouth, none of these work. Today I tried again but she wouldn't stop so I rolled her over on her back in the gr............... she walked well after that. It probably won't work next time though. Is she doing this because she doesn't want me to tell her where to go? How do I stop it? SHe doesn't do this when I talk her on a different street or route.