During my first trimester, I experienced occasional pain on the lower left portion of my abdomen. It is like a stabbing pain and I usually encounter this when I stretched or when I have been walking for awhile. Sometimes it happens all of a sudden. Though it only last for a minute or two, I am quite concern. I do not have any infection and now that I am on my 24th week of pregnancy, it doesn’t happen too often. My doctor can't find anything wrong with me. Why do I experience such pain? What is this all about?

Thank you for your help.