5 week ultrasound

I’m curious how my HCG levels doubled overnight when they hadn’t even doubled the last beta I did. I had an ultrasound and only a single gestational sac with the very start of a yolk sac was present. The doctor said with the huge increase overnight in my HCG I could be looking at multiples, even though only one sac was seen. Has this happened to anyone?

18 dpo (4 weeks 5 days) -1560

20 dpo (5 weeks) -2936 (DID NOT DOUBLE)

21 dpo (In ER, 5 weeks 1 day) -6332.4