Welcome to SR, I'm glad you found us.
When anyone changes that dramatically over a short period of time, there is obviously something going on. Yes, it sounds like it could be drugs, particularly with her taking anything without a prescription. Your guess is as good as mine, probably better, but what matters here is that she has made the break from you and is not reaching out for help.
It's sad to lose someone we care for, it's sad to watch them destroy themselves, but the truth is that there is nothing we can do to change anyone, particularly someone who may have an addiction. If love could save these people, not one of us would be here.
This is bothering you or you wouldn't be here. It's not easy to let go and it's not easy to move on, so there are no easy choices here, but there is the good choice to work on your issues and begin to heal and regain your balance.
Take a read around, particularly the sticky posts at the top of this forum. There is a lot of good information there, and a lot of people here who will be along to welcome you and share their experience, strength and hope.
Hugs