I really don't want to share my food with the other people in my house.
Why? Do they eat too much of it or something?
Yes, there are too many people eating my food in this house.
I get it. Your roommates may remember down the line that you didn't want to share, though. That may come back to bite you.
They don't need to remember, I'm only here till the end of the month.
You're not obligated to share just because you live together, although it really is polite to offer.