I do hope your MIL is feeling better.
My son had similar symptoms for 13 days - fever, fatigue, body aches, cough, aching ribs from coughing, etc. He was tested on Day 11, the results of which came back negative. Every doctor and medical professional that we have spoken with believe that despite the results, he was COVID positive. It seems as if 30-35% of tests are false negatives, and there are stories like
Nick Cordero who tested positive after 2 negative results.
I am not sure what would have been harder for us to deal with emotionally - if he tested positive on Day 3 or the uncertainty of the negative on Day 11. We will not know for sure until there is accurate antibody screening available. All that being said, he was 4 days fever free on Easter Sunday. While that is behind us, it will take him a long to to recover physically and mentally from this experience.