I have heard that some fibroids are more likely to give problems with periods than others, depending not so much on the size but on the placement in the uterus. You might request a uterine MRI from the other doctor, it will be a much better way to visualize what is going on.
Oftentimes, fibroids don't cause symptoms. But, depending on where they are, they can cause heavy bleeding. Those in the inside layer of the uterine lining (endometrium), called submucosal, are the ones that can cause heavy bleeding even if they are small. Taking hormones (estrogen or progesterone) as well as some foods or supplements such as soy can cause them to grow.
Of course, there are other things that can cause heavy menstrual bleeding, many of which don't tend to be checked by gynecologists. These include an endocrine abnormality (such as thyroid), a clotting disorder, or even the transition into menopause which can take 5+ years.
If the heavy bleeding causes low iron or outright iron deficiency anemia, that can actually worsen bleeding. So make sure your iron levels are adequate and supplement if they are on the low side. There is a non-hormonal medication that can be prescribed for heavy bleeding. You take it only while on your period. It is called tranexamic acid (brand name Lysteda). For some reason, it seems many gynecologists fail to offer it to their patients. Of course, it may not be appropriate for some patients but just thought I'd mention it.
Some women find menstrual cups (such as Diva Cup, Softcup) a lifesaver if they have heavy bleeding. I personally never used one.
I hope this helps!