To me it sounds like you did miscarry when I did I passed a round hard ball also, just because you had a neg test result from the dr does not mean anything when you are just preg sometimes it can not be detected I know two people who had neg results and where preg sorry you had to go threw this.
This is confusing. If it was an actual miscarriage, you would not have been far enough a long to have clots or fetal tissue. It's true that a Dr's urine pregnancy test may not always pick up an early pregnancy. Many Dr.'s use tests less sensitive then a home test. However, if you had had a very early miscarriage taking place prior to the urine test picking up the pregnancy hormone (HCG), you really wouldn't have had more then just heavy period like bleeding. I have had 3 of them (called chemical pregnancies) and if I didn't know better, I would have thought a late and very heavy period. I am wondering if you didn't have a kidney stone? I think you are wise to discuss this with your Dr. I really don't think this sounds like a miscarriage. Good luck.