chronic lower abdominal muscle pain

Hi, am wondering if anyone can tell me about the chronic muscle pain I have had for 3 months now.  I am 48, in good shape, play hockey once a week and exercise moderately.  I have had a muscle pain which is below my belly button into the pubic area only when doing sit ups.  I thought it might be a hernia but there is no bulging or pain to the touch externally, only internal muscle pain when trying to sit up.  Any ideas as to what it could be.

Thanks!