Miscarriage?

I woke last night with severe cramping in my lower back and side. It was too the point to where I couldn’t breathe because of the pain. I woke my boyfriend to take me to the ER because I figured that I was having a miscarriage. We got there and they did a ultrasound on my kidneys and bladder. They took my blood and did a clean urine test. They did a ultrasound to check the baby. They told told me that my kidneys and everything looked fine. My white blood cell count was normal so there was no indication of a a infection. All test came back good. They said that since I wasn’t bleeding then it was unlikely that I was going through a miscarriage and that my HCG levels were extremely high. The only thing the doctor said was the baby was measuring 5 weeks 2 days. Even though I am supposed to be 6 weeks and 1 day. They also wasn’t able to detect a heartbeat yet but we saw the sac and everything. But he said because it’s early then it’s normal. I’m just concerned that maybe this is a missed miscarriage or the start of one. Anyone had this happen to them?