Can't checkout new branch

[bmacq]

I created a new branch in bitbucket, and so at this point the new branch and the existing branch it's based on are identical, and both branch labels point to the same commit.  But I can't figure out how to get Sourcetree to checkout the new branch. When I try to checkout, it just checks out the previous branch label.  I know how I could do this in git bash but trying to learn Sourcetree as well. I also realize I could do a Clone with Sourcetree from Bitbucket, and get the option to checkout the new branch, but I feel there must be a simpler way. 

Version 3.4.3