Pi phase shift b/w adjacent pixels while using fft2

Hello,

I have a 2D Gaussian beam at its waist where the phase front is planar. I use fftshift(fft2()) to calculate the spatial 2D Fourier transform, and I observe that there is a pi phase shift between adjacent pixels. The figure shows the phase plot of the Fourier transform. However, when I use for loops to calculate the discrete FT, I do not observe this. Is this due to phase wrapping? How do I overcome this?

Thanks.