This is all Marianne going to extremes, as she always does. From the first chapter, we are told that Marianne is "eager in everything; her sorrows, her joys, could have no moderation." All of this 'changed' behaviour is Marianne showing no moderation in trying to atone for how badly she hurt Elinor.

Marianne has always wished to avoid doing anything that would hurt Elinor, and has, long before now, deliberately toned down or subdued her own outward expression of feelings in order to spare Elinor's feelings.

Back in Ch. 4, when she and Elinor are talking about Edward's taste, or lack thereof, " Marianne was afraid of offending, and said no more on the subject". and "She would not wound the feelings of her sister on any account, and yet to say what she did not believe was impossible."

She is still, at this point in the story, trying not to wound Elinor's feelings, but neither is she saying anything she doesn't believe. Her concession is to keep quiet when she might otherwise have allowed an outburst, but again, this is not because she doesn't feel like having an outburst--as evidenced by the throat spasm and moving to a chair farther away from Mrs. Jennings.