Why did Leah first say "Hashem saw," and afterwards "Hashem heard"?
Malbim: "Hashem heard" applies when a person knows his affliction, and prays. "Hashem saw" applies even when he does not know his affliction. Initially, Leah did not know that she is hated, just she is loved less. After she gave birth, and still Yaakov did not esteem her like Rachel, she knew that she is hated, and prayed about this. 2
Hashem saw that she was Senu'ah and she conceived the first night, before she prayed. Shimon was conceived after she prayed. (PF)
Ha'amek Davar: It says that Hashem saw, for Yaakov acted with her like other men, so others did not know that she was hated. Alternatively, Hashem heard my prayer, for I am hated, and Hashem seeks [to help] the victim.
Why did Leah say "Gam Es Zeh"?
Tosfos ha'Shalem (2): A twin sister was born with Reuven and with Shimon.
Why did Leah first say that she is not loved, and afterwards that she is Senu'ah?
Ohr ha'Chayim: At first she thought that she is not hated, just she is loved less. She thought that her first son is to enter love. When she had another, she realized that one was to remove hatred, and the other is to enter love.
Refer to 29:33:151:2.