Help — my dog is suddenly very skeptical of my boyfriend

My boyfriend of a few months and I are long distance and hes visited me twice since we started dating. The first time he and my dog (a 1.5 year old Bernese mountain dog) got along very well — she’s a dog who gets along with everyone, including the mail man. They were like best friends and couldn’t keep their hands/paws off of each other. But this time she doesn’t seem to remember him at all, which is odd, and barked and growled and will do anything she can to stay on the opposite side of the room as him. He came to my bedroom to hang out (a place my dog rarely goes) and we found her watching him in the doorway. She follows him through the house but keeps her distance, surveilling him. When he left she practically escorted him to the door and then hopped up on the window seat to watch him go through the window, like “that’s right buddy, get out of here.” He’s not abusive or large or menacing in any way and dogs typically love him? What’s up with this? Why the change of heart? He was excited to see her again and he’s been giving her treats and throwing her ball around for her but she remains skeptical and mistrusting. Is she on to something?

P.S. we both got tested for covid before he visited to be safe, so maybe because of the pandemic she’s not used to having anyone besides my family in our house? But it’s weird that just two months ago they were practically bffs and this time she’s wary of him.