Possible twin?

Okay I’m 4 weeks going on 5 well my levels were 1031 then suddenly stopped going up fast while I was 4 weeks and I had an ultrasound done yesterday and one sac is still growing from the last time, but they also “may have seen another sac” they weren’t sure, can that be why my hcg levels recently dropped on the 13th to 500, I still have no bleeding, nor cramping, still have my pregnancy symptoms as well, sorry I’m just so confused