Although the fall about 2 weeks earlier could have potentially caused rib fractures, your history does not suggest it very much.The pain due to fracture of a rib will usually aggravate on breathing in, or while coughing or sneezing.
You seem to be experiencing pain in the vicinity of the left rectus abdominis muscle(the "six-pack" muscle). One possible explanation for your symptoms is a partial tear of the rectus muscle. This could have been aggravated by by the dirt bike riding or some other activity/posture.
These tears usually are managed non-operatively(rest, analgesics and cold compresses in the acute phase). In some cases, if accompanied by a large hematoma, then they may need an aspiration or open drainage.
It will be safer for you to show it to your family doctor and get treated.
MB BS, MS, FRCS(Edinburgh)