Possible miscarriage

I'm 6 weeks and am bleeding red and have cramps. I went to the doctor today and they did a vaginal ultrasound and didn't see a sac or baby. The doctor said it can be too early to tell. They took my blood and I have to go back in 48 hrs and see if my HCG levels increased. If they don't he said I miscarried but if they increase then it's possible I'm still pregnant. Has anyone went through this?