Pharaoh was very upset by Moshe and Aaron's request that he told his task makers to no longer provide the jews with straw to make bricks. He would make the jews collect their own straw. He did this to spite the jews.
Pasukim Yud Tet - Chaf Gimmel
Pharaoh's task makers go to the jews and tell them that they will no longer be given any straw to make bricks with. They also inform the jews that although they won't be provided with straw their work load will remain the same. If the Jews did not complete there work load they were beaten.
The Jewish Foremen went to Pharaoh to ask why he is doing this horrible thing to the Jews. They ask why he is not providing them with straw and yet still want them to do the same amount of work. Pharaoh tells them that he is doing this because the Jews are strikers and that they must get back to work and accomplish the work in the same amount of time they would have if they were given straw.
After the foremen of the Jews went to Moshe and Aaron saying that they should be punished for what they have put upon the Jews, Moshe and Aaron ask Hashem why He has put such harm on the Jews? They wanted to know why ever since they came to Pharaoh, Pharaoh has acted even more cruelly to the Jews.