Measuring behind...brown spotting

Michelle? • I have a 3 year old son and just got my bfp for baby #2!!!❤️❤️❤️
Hi ladies,
Yesterday I went into the dr for an ultrasound and was supposed to be 7-8 weeks based on lmp. I was only measuring 5 weeks but heard a healthy heartbeat. Dr didn't seem concerned although I am a bit worried. Today I woke up with brown spotting which the dr said could happen but why isn't my mind at ease? Any experience or words of encouragement would be great. Thanks