A friend of mine was attending an exercise class for about a month where everyone was very congenial and friendly and joking; All of a sudden one day when she attending everyone was very businesslike and acted as if they did not want to speak to her. She asked me about it, but I was as perplexed as she was. Any thoughts ?Thanks.

I don't know, If you are a boy then may be all girls got jealous to see her with you. ;)

It doesn't matter what anybody thinks of anybody else, it only matters what you think of yourself.

I would leave it at that.

No doubt there is a 'ringleader' who has sheep followers, and who needs either one.

Continue to be a good supportive friend, and encourage your friend, to rise above it.

I would say try to ask the people why they acted that way

Perhaps your "friend" did something or said something that the others did not like. Honestly, if none of us were there to see it, there is no possible way that we could give you an answer for this as it could be anything.

Chalk it up to a bad, weird day and see what happens.