It depends. Did it happen intentionally?
If yes, then she's weirdly needy and emotionally unstable, and you should run the other direction -- unless you want a messy, clingy relationship with a chick who slings her boobs on top of men to get attention.
If no, then she's probably embarrassed, and you should be a gentleman, and pretend it didn't happen.
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It could mean that she trusts you and feels comfortable with you, but it's important to communicate openly and respect her boundaries. Not all physical actions signal the same level of interest, so make sure to have a conversation about what each of you are comfortable with.
It is signalling a fair degree of intimacy - but not necessarily inviting the next step.