Why does my dog limp after walking?

I have a 3 yr old dog. She limps after walking,playing, or accidently hitting her foot. It has been happening for a few months now. Some days she's fine, others she's in pain. One night it was really bad she was crying when she moved it a lot. Its only her right back leg. What could it be? And can I buy anything to help?
Asked Jun 11, 2015

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