Symptoms Come And Go - Miscarriage?

I wasn't sure which topic to post this under but here goes. my LMP was July 9th so that should make me at 6 weeks pregnant. I had some concerns because i was a little crampy and I spotted on Aug 8th after an internal and pap smear my obgyn did. I was in for a routine exam and mentioned being a day or two late on my period so he ran a blood preg test and it turned out positive. That whole week my breasts were extremely sore and tender then this past week they felt completely normal. The last few days they were sore on and off. This is my third pregnancy and by now I was always feeling tired and nauseated, but I am not. I had 2 hcg levels done. Aug 16th it was low 2000s and Aug 18th it was around 3700. Today I had an ultrasound and they saw the yolk sack but it measured at just under 5 weeks which makes no sense to me. I should definitely be at 6 weeks according to when i had sex. They are doing another ultrsound next Tues. Could it be the baby stopped developing at 5 weeks? My breasts are slightly tender today, but that is my only symptom - like i said it comes and goes. I'm not crampy at all. I'm so nervous because I really want this baby and all I can do is wait until my ultrsound next week to see if the baby grew at all. Are my coming and going preg symptoms a sign of miscarriage?
~Melanie, 32 years old