how much of this is just an adjustment period?

My husband and I have a friend’s dog who is with us on a temporary long term basis. We’ve known her for years, she is 5yo and well trained, very sweet. She has been with us for 3 weeks now. She has begun exhibiting behaviors our friend hasn’t seen before, especially resource guarding. She only guards against other dogs, NOT any humans. The thing is, sometimes there is no discernible trigger. All food and toys are removed, everyone receiving affection, etc. She seems on edge and it makes me anxious as I have a 10 month old baby. Luckily, when there are no other dogs in the house she does NOT exhibit these guarding behaviors! My question is, how much of this is an adjustment period and her becoming comfortable in our home? Should we expect it to continue? We have a couple friends whose dogs have separation anxiety so they are accustomed to bringing them over. For the time being, we’ll ask them not to. If we want to think about allowing it again, is there any chance that it could be okay? I’ve read the wiki’s on resource guarding and dominance which seem to be mostly about dog/human interactions, which this girl doesn’t have a problem with.