Rashi and Rashbam: Based on the word 'Pasil,' she meant that she was joined with her sister ¹ to merit children.

Seforno: She meant that she was intimately joined with her husband with a Divine and holy union, because, together with her sister, they gave their maidservants to their husband in order to bear the tribes of Yisrael. ²

Targum Onkelos: She beseeched Hashem in prayer, and He answered her, and gave her a child, like her sister.

Da'as Zekenim: She suffered great pain.

Tosfos ha'Shalem (5): I strove to overcome her, and I succeeded.

Tosfos ha'Shalem (9): I was crooked with Simanim (like 'ha'Chosemes and Pesilim' (38:25) were Simanim) with my sister [to help her fool Yaakov], and succeeded. A Midrash says that when Yaakov married Leah, Rachel was under the bed, and when Yaakov called "Rachel," she answered.

Naftulei means covered, like "Tzamid Pasil" (Bamidbar 19:15). There are Divine secrets covered from people why Leah gave birth and I did not.

Seforno: She meant that, with the birth of Bilhah's children, she had achieved what she set out to do by giving Bilhah to Yaakov.

Da'as Zekenim, Moshav Zekenim, Rosh: This implies that she exceeded Leah! One Chacham builder will come from Leah (Betzalel), and two from Bilhah - Ohali'av from Dan, and Chiram (who finished Bayis Rishon) from Naftali (Melachim I 7:13-14).

Malbim: I was able to bear [b'Simchah that Leah gave birth and I did not, for I know that Hashem is Tzadik in all His ways].

Ha'amek Davar: Also I can [be the cause of many sons, like Leah, for she ceased having children, and Bilhah can bear many].

Oznayim la'Torah: Rachel figured that, since Leah had stopped giving birth, and Bilhah already had two sons, Bilhah would give birth to another two sons - four like Leah. Leaving her to produce the remaining four, ¹ thereby giving her the edge over Leah.