Hopefully your back pain goes as quickly as it arrived. My usual advice applies, which is:
- Keep it mobile but don't stretch it out
- Apply some Arnica Gel at least twice a day
- Take some extra magnesium if the muscles are tense.
Regarding the pain under the left rib cage, it might be indigestion (believe it or not), as most of us sit a little slumped which can allow stomach acid to cause a warming/burning sensation.
Try eating little and often, making a conscious effort to sit upright (as you say, it goes when you stand up) and if necessary, try a digestive bitter such as Centaurium before eating.
It may alternatively be another secondary injury sustained when you hurt your back (even a cracked rib) or lastly, and relatively unlikely, your spleen is unhappy.
The location you've described is exactly where any pain from that organ would refer itself to. In that case, you'll need a trip to the Doctors.
Hope the answer is a simple one and your back to yourself in a few days.