What can be the cause for hemoglobin in urine?

So it’s me again, I’m 6 months pregnant, woke up this morning after doing my fast – no water no food for 12 hours for the gestational diabetes test only to find I peed blood and only blood so it seemed.
So I’ve just spent 4 hours in hospital, the Dr said she didn’t think it was an infection as their were no red blood cells but hemoglobin.
She thinks kidney stones, of which I’ve never had any symptoms of.
I’m getting a ultra sound tomorrow to see if there’s anything there, I’m worried about cancer as my Nana had kidney cancer.
Any ideas?
Could it be from dehydration, the first two samples were red then the next one was almost clear – she said the second one only showed traces of protein and blood – but first was quite thick.

Thanks so much.