I haven't had this problem but am copying a few lines from the NHS website relating to periods:

"Your periods can change, for example they may last longer or get lighter.  This doesn't necessarily mean there's a problem but it does need to be investigated.  You can go to see your GP or you can visit your nearest women's clinic or contraceptive clinic." 

I realise you have already seen your doctor and a gynaecologist.  I am only bringing it to your attention as you might like to try your local women's clinic the information for which should be available under the NHS information website.  It might be worth waiting for your further ultrasound but there is a chance that  somebody at the women's clinic has some useful information about this.  Best wishes.  Annie