Does ra affect the back? first it was my ribcage, now my lower back is hurting too much, sometimes is difficult to be straight or sitting for long

It certainly can: See your rheumatologist to make sure there is not any other progressive issues or damage occurring. Discuss interventional options as well if you have either disc or facet problems, both of which can be affected. Also make sure they take a look at a joint called an si joint, or sacroiliac joint to see of there is any inflammation there.

Yes: Gererslly smaller joints are affected first. You could also have degenerative arthritis too. Ask your doc.

Yes: Usually we think of peripheral joints but it can be anywhere. Of course you may also have osteoarthritis or lumbar disc disease causing pain. Try otc meds like aleve (naproxen) or Motrin first. If no improvement go see your doctor.