Yep, it's normal to give in sometimes. It doesn't matter if you've just started or if you've been at it for a while. It usually hits totally out of the blue, too...

I've been fortunate in that I am pretty disciplined and it doesn't happen that often, but when it does.... Watch out!

Just pick yourself up and get back on track and you'll feel much better. It's always disappointing when it happens, but it's rewarding when you get right back on track and can feel proud of yourself for doing so.