Using the Langevin dynamics technique, we have carried out simulations of a single-chain flexible diblock copolymer. The polymer consists of two blocks of equal length, one very poorly solvated and the other close to theta-conditions. We study what happens when such a polymer is stretched, for a range of different stretching speeds, and correlate our observations with features in the plot of force vs extension. We find that at slow speeds this force profile does not increase monotonically, in disagreement with earlier predictions, and that at high speeds there is a strong dependence on which end of the polymer is pulled, as well as a high level of hysteresis.