A question about ibs?

Well I went to the doctors a few weeks ago and im 13. He told me that I had IBS..even though I only had one sympton which is ridoculous. So he dort of pre diagnosed me with he. I didnt even get any blood tests done to see if I attually had it. But im not to sure why they so quickly rule out IBS....? Maybe because it's attuually very popular or you don't need blood tests to test for it...im not too sure im only young. I went to the doctor again then I got diagnosed with pre mentural syndrome..?