Ultrasound concern?

I was in the hospital last week with preterm labor and kept overnight. The morning I was released they did an ultrasound to measure my cervix. They also checked on baby and measured her for size. They also said she has a perfect 4 chamber heart and looked at her brain. They said it all looks great. But a few days later my ob office called and scheduled another ultrasound and didn't say why. My ob said there was nothing to worry about when she saw me in the hospital. Has anyone else had an ultrasound a week after another ultrasound without being told there is an issue? Is it normal to have a lot of ultrasounds if you have preterm labor? I had it a lot with my daughter and never had an ultrasound for it. I thought nothing of it when they called but I'm getting nervous thinking about it.