Maybe pregnant again?!

Is anyone else here pregnant again? My husband and I have been super careful since we decided this baby was our last. Well I have been under a lot of stress lately so I didn't stress when I didn't get my period. I decided to take a pregnancy test for the heck of it and it came back positive! How can that be? We have been extremely careful and I have absolutely no symptoms! Has this happened to anyone else? Could it be a false positive?