There are a plethora of reasons you could be having this kind of pain...could just be an overly aggravated rectus or oblique muscle from your original injury, or you could have developed what's called a sports hernia.
Unless you've had any sort of previous abdominal surgery, i.e. laparotomy, not laparoscopic, having an incisional hernia is extremely unlikely. People have pointed out the possibility of an inguinal hernia, but if you're not having any groin pain, or noticing a bulge in that area when you increase intra-abdominal pressure, this is also unlikely.
Definitely best to see an internist and sort it out.
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