She's 21, you're 22. You're both going through a lot of changes - there is nothing wrong with being heterosexual (or pan, or homosexual, or demisexual, or asexual either), and it might mean that as she comes to terms with her own sexual and gender identities you two simply are not on the same path anymore.

And that sucks. It happens, often at that age, but it sucks.

The best you can do is be patient and decide if the direction she's heading is one you can follow. That's the best you can do, but you wouldn't be wrong for saying that you don't feel like where she's going is compatible with you and that you both need to move on either.

Don't make it about how feminine she used to be, don't make it about her sexuality. If you have to break it to her, just tell her you're growing in different directions and try your best to leave it at that. You absolutely should do everything you can not to worsen her current struggles, but you aren't obligated to see her through them at the expense of your own well-being either.