is Re-using Challah ok?

I keep the sabbath every week, and usually i buy or make challah. The thing is, i’m living in an apartment where I am the only jew. I don’t have a lot of money right now and i usually just rip off a piece of the challah for the hamotzi. Would it be considered unkosher to use last weeks challah a second time? it feels wasteful to buy a whole challah every week that barely gets used, but it also feels disrespectful to make kiddish over a blemished loaf.

Edit: After reading your replies and conferring with my rabbi, I will start making rolls instead of loaves and freeze them for future shabbats. Fear not, last weeks challah will live on… as french toast!