Definitely a white balance issue as others have said, a super easy fix in Photosop if you have it. Do you shoot and process RAW images? If so, when processing the raw file it gives you the ability to fix the white balance and it shows various options, as well as an 'auto' option that attempts to fix it for you. Auto is not ideal, but honestly it would only take you a couple mouse clicks to find the proper white balance when the sky looks 'correct' again.
I checked it out in Photoshop, adjusted the white balance and aside from it being a tiny bit 'soft', it's still a beautiful image. Keep at it!