Am I losing it or imagining it?

I have been slowly building muscle in my biceps these last few months. I have upped my protein and overall food intake. Now thist last week, I have not been able to eat a normal amount of food. I got out of the shower today and I looked at my arms and they looked visibly smaller. Now I know that if you do not eat enough, your body digests its muscle. I was how long does it take for a starving body to breakdown muscle and use it for energy? A few days? or like a month?