Why is a my stray/feral becoming more aggressive?

I trapped a stray momma cat and her kitten about 6 weeks ago, and brought them into the house. They are currently living in my sun room (I have two other cats in the house). When I first caught them, they were very scared, and stayed quiet and hidden. Now they're a lot more confident, and I can even get the little guy to play. However, the momma, Clotho, stands by the door and hisses every time I come into the room. Usually, she'll hiss and then run and hide, and then come out when she decides I'm not doing anything too threatening. Whenever I come in with their food, though, she'll do a really aggressive display: butt up, head down, front legs splayed, hiss and growl. This morning she actually took a swipe at me and got me as I was setting the food down. I understand she thinks the room is her territory now, but why the aggression around food? And any ideas what I can do about it?

Do you tend to spend more time in there when you bring food?

She is more comfortable but doesn't seemed to have recognized you as the bringer of food yet. I would approach quietly and slowly. Have you tried blinking slowly at her? This is a warm smile to a feral, so she will come to understand you are her friend.

If she hasn't been spayed yet, do so soon. She may be pregnant already, which could be making her testy. Fingers crossed she isn't.