Cramping.

I experienced cramping yesterday which led to an early ultrasound at 3 weeks and 5 days. I know it’s way too early and nothing can really be seen in an ultrasound but they couldn’t find anything and they seemed concerned and told me if it got worse to go to the ER. They left me really anxious. They said my Ob would explain tomorrow. And now I’m having cramps on my lower back. Could something be wrong? There is no bleeding at all just cramps.