Were you exposed to more sunlight than usual this year? I agree with puerkz, bicarb can't lighten hair. It has been a hot summer everywhere as far as I can work out, I'm in NZ so I'm just coming out of winter so going by what friends and family say! This could mean you've been more exposed than usual purely from the strength of the sun.
I second lime juice or lemon juice, but it still requires the sun to do the lightening. Just please, don't use a spray in product like Sun In. They're REALLY damaging.