HcG levels

So on September 25th my levels were 75, putting me at 3 and a half weeks. I went in today, like a week later, and she told me as long as my levels were about 300 then my pregnancy should be progressing normally. She called later telling me my levels were higher than expected, at 2,250 😰😂 how did they go from 75 to over 2,000 in a week?! She said I'm probably just a week and a half farther along, but my husband is now convinced that it's twins. Anyone else have this happen or have an explanation for me? I'm so confused.