Adopted dog help

A few days ago my family and I (me, partner, 5yr old kiddo) adopted a dog thru a shelter. Almond, two years old. So she spends a hour or so with us just chilling at the shelter while we get paperwork filled out, no real issues there. Bring her home, she's fine, if just a little stand-offish. Eats and drinks little, pees/poops once a day, but we assume it's nerves and will sort itself out. We kennel her overnight (something we're hoping to eventually stop, we just wanna know she won't get into anything she's not supposed to). This morning I'm heading downstairs to pee before taking her out, and she's growling. I think maybe it's a stray cat outside, no biggie. Toilet her, bring her inside, and she is more stand-offish than usual, staying away from everyone and growling at something. Def not towards us, but nothing we can tell. So we gave her space, let her sit at her usual area until we had to leave for school/work, but she just seems off. What could be up? I'm concerned because she hasn't acted like this since we got her, and we wanna help her be comfortable.