Continuity mistake: In the scene where Eve is contemplating mating with Dennis Gamble in her lab, you can see her in the door's reflection standing. After rejecting him, she sits is a chair and turns away. The following shot shows Dennis's reaction to be rejected, but also show Eve's reflection still standing.
Continuity mistake: In the scene where Eve breaks out of her glass room lab, she is wearing a long green summer dress that is open almost from the ground to her waist revealing white underwear when she jumps down from the lab. In a later scene with Patric Ross at the barn she unbuttons her dress, steps out of it and is naked, sans underwear.
Continuity mistake: When the Debutante begins to examine herself in the mirror the jewel on her necklace is missing, it appears again when she reacts to her stomach expanding.
Answer: She was most likely giving herself a breast-self examination. Doing this would enable her to feel for lumps, distortion or swelling which if felt early would mean that she might have early stages of breast cancer and could get it checked immediately.
Her breast was sensitive and painful because she could sense that something wasn't right with her.
So the alien embryo inside of her body was giving her cancer?
No. It wasn't giving her cancer. She probably does a breast-self examination periodically to insure that there's nothing wrong with her breasts.
Plus, it wouldn't make any sense for an alien embryo to give her cancer since after giving birth to an alien hybrid she would immediately die.