Wellies - the following is easier to say than do!
However you should be concious that your soft reassurances may (may not will - I don't know your pig or you) be actually making her worse - any hesitency and stress in your voice and body actions could be increasing her anxiety, as she picks up on your concerns, thus heightening hers.
I would go in with a pig board - this gives you something to put between you and her, and thus should give you confidence to approach her at a normal speed as you used to do and speak normally to her in a friendly confident tone. If she lets you scratch, give her a quick scratch, and then stay in the pen and do something else - possibly greet and scratch another pig. That way you are not singling her out for unusual attention, and are making your confident presence in the pen a normal activity.
If she backs off from you, then ignore her and again do something else, don't persue her as that will stress her more.
The quicker you show her that she is not singled out, and that you are not worried, the quicker she is likely to return to normal. The more you make her mood an issue the more likely it will remain so.
If you don't have a pig board, improvise one, a handled dustbin lid can be fine, all you want is somnething to put between you are her to give YOU confidence.
Let us know how you get on