My hCG level is at 56, 000 today, I'm only 6 weeks 3 days pregnant. Could it be twins?

It can vary. The HCG level can vary considerably, especially the farther along a pregnancy progresses... I've seen high levels in singletons and low levels in twins/triplets! most published studies suggest one can not predict the number of babies based solely on the HCG level, especially when someone is in the sixth week of gestation. Congrats!
Maybe. It could be twins; it could be a normal single pregnancy, and you dates are just off by a few days; there could be a problem with the pregnancy. At 6 weeks, 3 days pregnant, normal HCG can be as high as 43000, but it can be up over 57000 just 2 days later. If you are worried, talk to your dr. Good luck!