Not Sure

My mom and Diane are both divorced and have been best friends for years. A couple of years ago they started an online business and set up an office in our house.
Back at the beginning of this summer I came home and saw the two of them naked together in the office. Of course I was surprised and so were they, although they just laughed about it and said how hot the weather was.
I kind of didn’t know what to think exactly, except that they felt like having their clothes off. I started to wonder though if there isn’t something more going on between them. I’ve been afraid to ask, and wonder if it really is any of my business. It seems to me that my mom would have said something or should I ask?
Has anyone else found their mom in some kind of female relationship, and how did they find out?