What could cause moderate pain in chest that has been constant for 5 weeks and started with a lump under my left rib that has since gone?

Many things: It is pretty clear that this is not coronary artery disease (heart attack) due to the duration of the symptoms. A lump may be caused by: hematoma (bruise), abscess (infection), or even malignancy. If the lump went away on its own, not likely to be malignancy (cancer). A bruise or infection is more likely. There are some odd inflammatory processes, but i don't how they would correlate to this.