Factual error: During the Mars-flight, the way the spacecraft uses the rocket thrusters is unrealistic. For example when they are nearing Mars, they should be using the rockets against their movement, to slow down the speed. But in the movie they are using the rockets towards Mars.
Factual error: Crew members aboard the spaceship at the planet Mars are shown in instant communication with Earth. Because of the distance, a few minutes must pass each time before communication is made (between 10 and 22 minutes depending on where the planets are in their orbits, and that's each way so at least 20 minutes from when you say something until you hear what they say back).
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.