Since you've just gotten her, give her a week to get her bowels sorted out. She probably just has to get used to eating more frequently. Her metabolism will be slow from store feeding so it'll just take some time.
I've got two new bettas right now. the female I bought two weeks ago is still having very minor sbd when she eats, but she's improved greatly. The male, which I've had for a week now also still has sbd issues with every meal. He's also improving. It just takes time.
If she's having a lot of trouble staying upright, then you can start epsom salt at 1 tsp/gal for a week minimum (Just to make sure everything gets cleared up so you don't have to deal with little flare ups).