Hi My Mom Is 74 Years Old. Recently, She Has

Hi My mom is 74 years old. Recently, she has been complaining of pain in her thighs. She can barely get into my car (she said the seat is too high), and she can not get into her bed without using a step-stool. Sounds crazy. About 3 weeks ago she was recovery from some sort of a virus. She was very achy for a while, but has not fully recovered - what test does she need to take to find out what she could have?? Could it be RA??