If you feel constrained to do something, you feel that you must do it, even when you would rather not. Manners, a rule, a job, or other people’s eyes are pressing you. “She felt constrained to apologise” means she felt she had no decent choice but to say sorry. It is rather formal, and a little old-fashioned; in everyday talk people usually say “I felt I had to” or “I had no choice.” You will still meet it in books and careful writing. The same word can describe a manner that does not feel free: a constrained smile looks tight, as if the person is holding something in. The feeling underneath is similar — you are not acting from a fully open heart. Constrained is the -ed form of constrain, “to limit or force.”