Concerned!? But should I be?

Yesterday I went for my first ultrasound at 5 weeks. There was a sack but nothing else. The doctor said it could just be too early to see anything but scheduled me to come back on Wednesday for another ultrasound. I have had no spotting/bleeding. I have had some cramping and tender breast at times. I am feeling a bit concerned/scared! Should I be? Anyone else have this happen? Any advice welcome!!