MIL-what could this be?

MIL went to the dr on Friday. She was having severe pain, she said her whole body felt like she worked out really hard. All her joints and muscles were just sore and achey. The dr took blood work and said it sounded like she had a virus, well they called her yesterday and told her it wasn't a virus that it's something else but they aren't sure what and to call back in a few days if she still isn't feeling well. It was hard for her to open anything and use her hands or pick up anything or to even shower, she's feeling a little better today but still just very achey. What would cause that?

arthritis?

My Grandma feels like that a lot & she has arthritis really bad.