Queen of the Orcs: King's Property by Morgan Howell. It's the first book of a trilogy (the others two books are $5.59 each on the Kindle)
From Booklist
*Starred Review* Conscripted into King Kregant's army, Dar is branded to make running away difficult. Assigned to serve food to the orc regiment, she discovers that they worship Muth la, the mother of everything, and can take food only when served by a female as well as with a ceremonial phrase indicating that it comes from Muth la. When Dar realizes that one of the orc soldiers' human leaders has it in for her, she turns to the orc Kovok-mah for protection and attempts to learn his language, which makes her an outcast from the other women in the army, many of whom service the soldiers in return for sharing the plunder. When young Twea is conscripted, Dar takes her under her wing, and Twea enchants Kovok-mah. Using logic and chutzpah, Dar keeps herself and Twea safe, however, by sleeping in the orcs' enclosure. While huge and strong, the orcs are straightforward and honorable, and their language reflects their lack of guile. They are fighting because their queen requires it. Howell has created a fascinating, believable world and a compassionate, kick-ass heroine who helps the weak and uses wit and agility to survive and triumph. Tixier Herald, Diana