Mother-in-law agrees to watch 3 year old who has a fever. You tell her you don't know what she's brewing. MIL decides to test 3 year old behind your back and gets mad about COVID

My daughter had to be home yesterday due to a fever of 38.1C (100.6F) overnight. She woke up because she was too hot and I took her temp. She took some Tylenol and had no other symptoms. She is now symptom free and not taking any meds since that Tylenol dose.

**Update: of course I usually stay home with her but because she was otherwise fine and my mother-in-law literally lives three doors down from us, we asked her. We only get so many sick days and want to save them for when we really need them! End update**

My husband and I called my mother-in-law yesterday and asked if she would watch our 3 year old and she agreed. My mother-in-law asked if it was COVID. I said (truthfully) "It could be anything at this point. If it is COVID, are you still ok watching her? She only has a fever, but if she gets worse, I'll take her to the doctor." She said she was fine watching her.

I gave my mother-in-law masks and hand sanitizer. My mother-in-law decided not to wear a mask because it was only a fever.

An hour after I left, she called me screaming about how my child has COVID and now she's infected her. I told her to stop screaming at me. I reminded her that she agreed to watch her knowing that it MIGHT be COVID. She said, "No, she HAS COVID. I tested her. If you had any sense you would have tested her before I got here." I told her we don't have rapid tests anymore and I reminded her that she knew that this morning! I told her this morning that it could be anything! ***Side note that she decided to bring rapid tests (didn't tell us at all) and then tested our daughter without our consent. Rapid tests haven't been available for almost a year where we are.The rapid test she brought also expired in February 2023 so how accurate are they? This is a whole other argument my husband is having with her because that's not ok that she tested her without our knowledge.***

Now she's leaving nasty messages saying we did this on purpose and I just can't deal with her. Now she's saying "Well I said I was ok watching her if it MIGHT be COVID. I never said I was ok watching her if she ACTUALLY had COVID."

My thought is this: you knew your grandchild was sick. You knew there was a chance it could be COVID. You said you were ok watching her knowing that. You decided to test her without our consent. I fail to see how this is on us at all!

So... Am I the asshole here?

Edit: IT WAS A FALSE POSITIVE! The day after this happened we went to the ER because my daughter tripped and couldn't walk properly afterwards (she's fine, just bruised her knee pretty badly). I told them when we arrived that she tested positive on an expired test just yesterday. They did two tests. She came back negative on their rapid AND negative on their lab one. They told me that the rapid from yesterday was a false positive. My mother-in-law called us up that evening and said she was now positive and she was still mad and yelling. Once she let me get a word in, I told her about how it's probably a false positive. She gave me one of her rapids and I just put the solution on the test (didn't swab myself first) and it came up positive without even swabbing. I told her the tests are bad and giving false positives.