Taylor Swift appeared on The Tonight Show on Wednesday night, but she wasn't exactly herself ... much to our delight, the singer brought back her nerdy Natalie character -- and brought the laughs with host Jimmy Fallon.

The duo starred in the show's "Ew!" segment, and from dancing to Selena Gomez's "Come & Get It" to sneezing on Fallon to talking about ex-boyfriends, Taylor never missed a beat. When asked about getting back together with a former flame, she responded, "Ew!" and when Fallon asked, "Never?" she replied, "Never, ever, ever, ever, ever!"

Ha! Click the video above to watch her geek out, bust some moves and act incredibly awkward...