Hey, Bliz: are we related? Because except for the medication, you could be describing my daughter (your niece). Body changes, weight loss are perfectly normal at this age. Losing baby fat and changing shape are all normal. I know medication is a concern, but kids not on meds do exactly the same thing.
My ex would make comments about my daughter's apparent "pudginess" for a few years...I was adamant at drilling into his thick skull that he should NEVER comment on that to her, that this is normal, etc for her age, I even sent him papers out of medical texts to shut him up. However, it was clear to my daughter that dad didn't approve....but she had me to vent to and go for acceptance. Now things have changed again and she is taller, thinner and looking like a grown woman. I notice boys and young men look at her. Now my ex is all freaked out that she looks like a woman! HE will never be happy with his children.
As far as the not thinking adults are cool, or certainly not wanting to cuddle like a little kid, this is also very normal. Behavoir changes, trying on attitudes are all part of it. It is the teenager's job to become autonomous. Absolutely essential to survival and growing up. As long as they know they CAN get a hug when THEY want, or can have a safe place to act babyish at times, that behavoir is perfectly normal and expected. A great book on teenagers is by John Wolf> called "Get Out of MY Life, but first will you drive me and Cheryl to the Mall?" the title sums it up. The book is great for anyone who has a teenager in their life!
My daughter is also wound rather tightly, straight A student, very active in other activities, teachers think she's great, etc. I do believe her perfectionism may be in part to walking on eggshells around a very N dad, but it's also her nature. She has recently started trying out different "looks", different friends, all within normal. I keep my eye out, put my foot down on occassion, but she needs to do this, with safe back up (me).
Don't take it personally when she rejects you. You are an adult, therefore not too cool. The "closer" she felt to you in the past, the more rejection you will get.....so be complimented! This doesn;t mean they get to treat you like crap...."I don't appreciate that comment" is perfectly acceptable....but asking them to be cute and little again is just pointless.
She will respect you, but don't expect it to be the way it was. In a few years, you'll be the cool aunt again. Lucky kid.