Sid, your doctor really doesn't know much about HRT does she? A gynaecologist who knows what they are doing would not panic about a 7mm womb lining measurement (with no other abnormalities) on a woman who is using HRT. They would prescribe a heavy duty progesterone and then scan you again immediately after the bleed.
If you have the financial means, it would be worth seeking help privately to get a second opinion.