What an interesting question. I can tell that you are more focused on your daughter's feelings and possible reactions, rather than on your own. As a mother myself, I do understand where that impulse comes from, but I would advise you strongly to step back a little and think about what it is that you want to do, rather than what she might want you to do.
Firstly, you are both adults. Yes, you are her mother but she is not a child anymore and as such, you both should be expected to behave and react in an adult fashion. Be open and honest with her. If you are sure that the profile belongs to her, then tell her. Explain that you are on Fetlife as well and that when you saw her profile, you thought you should say something. Be clear that you are not interested in her adult sex life - the last thing you want is for her to feel like you have been checking up on her.
However, if you aren't sure the profile belongs to her but merely have a suspicion, then you might want to test the water slightly first. In this case you might want to try steering a conversation towards the subject and mentioning the site to her. I am fairly sure that as her mother you will be able to tell by her reaction whether she knows it or not.
As a mother it is very easy to lose sight of your own life and needs - they often seem to pale in comparison to your children's. I don't think the instinct to do this stops when they become adults, but you do have just as much a right to explore your sex life as your daughter does. As her mother, it is your responsibility to demonstrate a mature way to deal with a tricky situation, by explaining this to her and offering her the opportunity to have a grown up discussion about it.
Of course, I know nothing about your daughter. She might find the whole thing funny, or perhaps even be happy for you. I can tell you that I know of a few mothers and daughters who are part of the same local kink scene. For the most part they are not involved in each other's relationships but do, on occasion, happily attend the same events. Not everyone would be comfortable with this, of course.
Only you can know what you are comfortable with, but I urge you to not feel embarrassed about the situation or silently slink away. It's not like your daughter is going to be able to be upset with you for being kinky or think it's weird or unnatural when she is already involved in kink herself. Hopefully this means that she'll have an open mind. There is only way to find out, though, and that is to talk to her. Take control of the situation quickly and with maturity and hopefully you can work out how you want to proceed together.
