BMI is a terrible indicator of obesity. All the toned, fit, muscular rugby players off the telly are obese, as they have so much muscle (which as we all know weighs more than fat).
I'm 5ft and somewhere between 125 - 130 lbs (it fluctuates quite a lot) - I'd say I was becoming noticeably chubby around 120lbs as I naturally have a very slight frame with very little muscle. Weight gain is so much more obvious on shorter people!