Fibroids are serious business. They can and do appear out of nowhere, and they can (though do not always) grow significantly during pregnancy (especially the first trimester) because they thrive on hormones. If you have not educated yourself about fibroids, do it now. The fibroid is responsible for your irregular bleeding. And the larger the fibroid gets, the more problematic it will become. There are limited options for dealing with fibroids. They are BENIGN irregular growths (tumors) of uterine muscle/tissue. They present in many different ways. They often complicate pregnancy (spotting/bleeding during pregnancy, miscarriage, pre-term labor, pain from a degenerating fibroid, obstruction of birth canal, forcing irregular fetal presentation, etc.), but they do not always have a negative impact. An ultrasound will not tell you accurately where your fibroid is located or what size it is. Usually the gold standard for that is MRI. During your pregnancy, however, your concern will not be with the fibroid so much as with the pregnancy. Get whatever information you can from your new OB/GYN about fibroids in general -- and about YOUR fibroid in particular (measurement, location and type). Be sure to write down any symptoms of the fibroid that you may have had up to this point. The irregular bleeding/menstruation. Other symptoms include changes in bowel habits, changes in urinary habits, abdominal/pelvic discomfort or pain, pain that radiates into the hips/legs/back, painful menstruation, bloating - especially PMS ... What women with fibroids often suffer is "symptom creep." You may not realize that your fibroid is causing you grief, until it really begins to run your life.
Start reading here, and see where it takes you:
http://www.fibroidsecondopinion.com/
There is also a Yahoo support group for women with fibroids ("uterinefibroids").
Could the fibroid obstruct the view so they can't see if you have twins in there? I think that's quite unlikely, given the difference in how a fetus and a fibroid appear in imaging.
Good luck to you!