Hcg levels rising but not doubling

1st and 2nd beta looked excellent and were more than doubling. Now, on third beta (7days from second) they are rising but now doubling. I'm worried sick. Our nurse said to be "cautiously optimistic" hard after being so excited and grateful to now fearing the worst.
I'm retesting hcg tomorrow and think I might demand and ultrasound. Anyone else experience this? Thanks