What are the signs of a miscarriage?

I am 5 weeks and three days pregnant, and I bled a little today with small cramps and tender breasts and some pinch in one of my breasts. I am getting some blood work done to check the “levels” of my harmonies because I went to a free clinic where they did an ultrasound and estimated my pregnancy 4-5 weeks at that time. I went two weeks ago, and this week I went to the doctor, and they did an ultrasound, and they are measuring me between 5 weeks and three days. They said I could be having a miscarriage, and my body hasn’t reacted yet, and that’s why I am still measuring the same, or the free clinic messed up with their weeks how far along I was. Has anyone experienced this? Any advice, please?