First timer at this

Just signed up today, question if I fill out an app for a better company would a dirty test that I took 7 years ago affect my chances of being hired? I drove for knight for 2.5 years been pulled in randomly for tests about 15-20 times, got to the point that I felt like I was targeted. My dp said that its just what the computer comes up with and that he even was called in for testing all most as much as I was as we on some acasions road down together . But this last time I went the doctor called me it was bad news, that he had too run my test 3 times because he had a hard time with it . He told me that I ate something that had THC in it still to this day I honestly don't know what it was. Been with this company for a little over a year and hate it. I've tryed some other company's one was so rude they just hung up on me. So if anyone can help with advice it would be veryappreciated.