Trivia: Co-star Michael Madsen, whom also appeared in the first film, has stated his distaste with this sequel, especially in comparison to the original. He once referred to it in an interview as a "crock of s***."
Trivia: An early concept for the film was to make a more direct follow-up to the original's climax, where we see a rat become infected by the alien DNA. The film would have revolved around an army of alien-rat hybrid monsters that must be exterminated. This idea was eventually dropped in favor of a different script.
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.