update: the vet thought they were spider bites. two of them were faded this morning (including the one I was concerned about) but the one by her front leg was swelled and inflamed. He gave her an anti-inflammatory shot and said to get some Benadryl if that was not helping.
He also recommended a bug repellent called Vector, but said since he couldn't be sure what bit her, that it may or may not prevent these bites. I thought about bombing the house next week when we're gone for a few days, but I haven't been bothered by any spider bites so I'm wondering if it's happening when she's out rolling around in the yard, as opposed to in the house.