Welcome to the forums.
The substrate you're using sounds like sphagnum moss. Personally I wouldn't use that for a pine, they don't require high humidity, so you might be better off with aspen or similar... maybe someone else who uses sphagnum can offer an opinion, personally I've only ever used it in wet hides for lizards.
The burrowing behaviour is quite normal. I used to keep a pair of Sonaran Gopher Snakes (another Pituophis) and the male would borrow regularly. If you only got her a few days ago she is just settling in, give her a good week without disturbing her. Then give her a feed, and a couple of days later slowly and gently you can start getting her used to you.
Hope that helps a little, just take it slow and realise that she is in new unfamiliar surroundings and needs a little time to adjust!
In the meantime do a little reading on the forum, lots of good info here, and do ask if you have any more questions.