yes of course someone should have offered you a seat - although I am always surprised when, on crowded buses, people with very young children tell their children to sit in vacant seats but continue to stand themselves (instead of taking a child on their lap).
People are always offering me seats these days, benefit of having grey hair (I don't think I look pregnant and am visibly beyond childbearing age!) and I always accept very gratefully unless I really am getting off at the next stop. Rude not to take a seat when offered, I think, as it can then be embarrassing for the offerer.