It is difficult to be sure of the cause of your pain. In pregnancy, the hormones that are present tend to lead to softening of the ligaments throughout the body and this makes a woman more prone to musculoskeletal pain (pain coming from the skeleton and its muscles and joints).
Pain in the region of the lower ribs is common in the later stages of pregnancy, particularly the last three months, when the womb is enlarging and starting to compress the ribs to some extent. This pain tends to be worse when sitting and somewhat less troublesome when standing or lying down. This pain tends to ease off when the baby’s head engages in the pelvis or when labour begins.
It’s unlikely that there’s a serious cause for your pain, but it would be a good idea to see your GP to try to establish what the cause is. Your GP will want to take a detailed history and to examine you to rule out other causes of chest and rib pain, such as a chest infection.
In the meantime, take care with your posture. Try to make sure that you stand up straight and that you sit in a good position with support for your lower back. Swimming is beneficial generally, and particularly so in pregnancy, especially backstroke, so if this appeals, then it would be a good thing to try, and is the sort of activity that you can continue throughout your pregnancy.