Question: When the Debutante examines herself in the mirror, why does she center on and support her breast just before doubling over in pain?
Answer: She could feel discomfort in her breast as she moved her hands down to her stomach.
Question: After an examination, Patrick, Anne and Dennis are told that as part of the quarantine, they can't have sex for ten days. Why? Excluding the fact that Patrick and Anne had been infected, what would happen to all three if they had sex before the quarantine was up?
Answer: Nothing would happen to them. The restriction on sexual activity was to keep them from spreading any potential contaminants through bodily fluids. It's nonsense because there are plenty of ways to spread bodily fluids that don't involve sexual contact and 10 days seems arbitrary, but it fits with the overall erotic themes of the film series to have the quarantine specify no sexual intercourse.
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.