I believe i strained my upper left back muscle, and upper left rib, towards the side when i was sick. i was coughing a lot and felt the pain. 3 month later i still feel the pain: i lift weights too, but only feel pain doing certain movements. cancer?

What you described is not suggestive of cancer. Keep up with the exercise. Aerobic exercise of sufficient intensity to raise your core body temperature will facilitate the healing "strained' muscles. You may wish to emphasize aerobic exercise over weight lifting till your muscles return to a baseline. Drink enough water so that your urine is mostly colorless. Wish you good health!

When someone moves and there is back pain it is likely musculoskeletal pain, from muscle, joints, ligaments, discs, or muscles. The best way to get an answer to your personal health question is to see a doctor in person. There is a very low chance that a 30 year would have cancer.