'16 and Pregnant' Star Whitney Purvis -- I'm Not Pregnant ... Period

'16 and Pregnant' Star Whitney Purvis -- I'm Not Pregnant ... Period: "16 and Pregnant" star Whitney Purvis tells TMZ ... she DID swipe a pregnancy test off the shelf at a Walmart this week ... but says she's glad to report she's NOT with child. TMZ broke the story ... the 20-year-old reality star was arrested in…