Tested positive for thc. Please comment

When I had my baby they said I tested positive for thc and I don't know how I haven't smoked since a few months before I even got pregnant so it wasn't even in my system. I told her that I drank but didn't know I was pregnant yet she said that could be it.. I was only about 3 weeks then if that's the case but don't understand how alcohol can come up as thc or even still be in my system.... Did this happen to anyone else ? Please comment