Now that I think of it, another possibility is if they didn't really do a hysterectomy and just told you they did. That's about as far-fetched a scenario as the other one, but I was just looking for any way a person after a hysterectomy could be pregs.
After a hysterectomy, they would KNOW if it was a double uterus, you would not have been told you "may have" a double uterus, someone would have said something definite. (Like, "Surprise! You did have a double uterus!") And certainly they would have told you if they did something as daffy as remove one half and leave the other. I mean, why were you having a hysterectomy if it was not to remove the uterus? They wouldn't have left any uterus in, if the point was to do a hysterectomy.
About the pregnancy possibility, the only way I can think of (and it's so far-fetched I don't think it has even ever happened) would be if you had an incompletely-sewn-shut spot where the uterus once was (i.e., so the sperm can get in from the end of the vagina). Then the sperm would have had to get in at a time when you were ovulating (I assume you still have one ovary or both). Then in this scenario, the sperm would have gotten in and swum up and now you have an ectopic pregnancy (because without a uterus, there is no other kind). There have been a few known cases of ectopic pregnancy in the abdomen, but the sperm always got in the normal way, up a regular cervix and up the tubes.
If you want to be sure, get a home pregnancy test and take it.