*[you cannot help her] except of course by encouraging her
I have a cousin whom I have often wondered why she didn't come out with some kind of attention-seeking personality disorder, because her mom (as it happened) was very dominant in her world, doing everything for her, including all of her thinking. (I was visiting them when she was about 14, and her mom asked her every three minutes, "Did you get your coat? Do you have your homework? Did you call your friend? Do you remember the practice is this afternoon?" After listening to this all afternoon, *I* was going crazy.) My cousin just took it very quietly and never once acted like it was unusual. I clearly remember that they were going to go to the store to pick up something my cousin was going to buy, and my cousin came back into the house because she had forgotten her purse. It was striking to me at the time as sort of symbolic of the whole situation that she hadn't noticed she had forgotten it but her mom had.
Please don't think that I'm saying anything like that about your parenting skills, but I did wonder where you said your daughter said she didn't know why she does it, if there was something sucking the oxygen out like that for her, when she was growing up. My cousin basically had to turn off the critical-thinking parts of her brain just to live with her mother (at 30, she was just finally working out that her mom had been a tad in her business when she was growing up). Maybe your daughter has a big envelope of unmet needs, and they cause her to try to get attention in that way.
Counseling sounds like the answer. At this point, for the purposes of taking care of her mental health, she is an adult (albeit a young one) and you really cannot help her except to be her supportive mom. She might decide to tell you what she discovers in therapy and she might not, but how she gets over things now is up to her. (Except, of course, by encouraging her to continue with a good therapist.)
Hi there and welcome. Well, your daughter does sound like she needs counseling. Munchausen Syndrome is something that happens in that she loves the attention. I am worried about her stability. She also does need to suffer the social consequences of it. Let her fake illnesses with friends who will eventually write her off as too troubled to be friends with. Sad, but she has to see how her lies and self created drama is damaging.
I'd beg her to do counseling. good luck