Pregnancy

I hope you girls read this and have feedback for me. I am currently pregnant. I went for my first ultrasound and was told my gsac measures 6w but they see no baby or fetal pole. Not forgetting to mention they were able to see two yolk sacs which might mean twins. They said I might be earlier in my pregnancy. I am irregular on my periods, my last period lasted from August 16-21. I took a pregnancy test the 27th and got a negative. I did another one on September 16th and got a positive. On Sep. 28th I got tested to see how my hcg levels were rising and they came out to 14,758. Then on Thursday I had to get it done again and my levels went up but only to 18,261. Should I be concerned about my pregnancy? Has this ever happened to any of you?