Insight? Only a gestational sac seen at 5w2d.. should I be worried?

I am currently 5 weeks 4 days pregnant. This is (fingers crossed) my rainbow baby. I had a miscarriage back in October and we have been TTC since February. I went into the ER 2 days ago due to severe right sided cramping (no bleeding). They did a v-u/s to confirm it wasn't an ectopic pregnancy. The radiologist confirmed a gestational sac that measured 5w2d but didn't see a yolk sac or pole. The doctor said it was likely because I was too early along to be able to see anything. I am still having mild cramping on and off and moderate lower back pain. I am worried a bit. Have any of you experienced anything like this? When were you able to see anything on u/s? Thank you!