Colonel Brandon arrives at Mrs. Jennings' the first night they are in Town. He is so shocked by Marianne's appearance and behaviour that it disrupts his typical gentlemanly behaviour and he doesn't even say hello to Elinor before asking "Is your sister ill?"
By Ch. 31, we know what he has been going through and how he has been wrestling with the decision of when/what/whether to tell the Dashwoods what he knows.
But the fact that he at first thinks her 'ill', then hears Elinor tell of her headaches, fatigue and low spirits, then watches her most intently, coming "almost every day; ... to look at Marianne and talk to Elinor"---this all makes me wonder if he at first feared that the same thing had happened to her as to Eliza?