Is punching in the arm/shoulder flirting?

There is this girl I kinda like, we are good friends and have been friends since middle school and know we are in college, and lately she has started to punch me in the arm/shoulder, but sometimes its pretty hard and she only does that to me even when we hang out. Do you think that's flirting or just being friendly?
Is punching in the arm/shoulder flirting?
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