My hcg level has been doubling ever 38 hours. Is this a sign I could be having twins?

May be. HCG levels vary widely in normal pregnancies and are not a very reliable sign of twin pregnancy. An ultrasound examination would be better for determining twins. You may consult this site for HCG level info.
HCG doubling times. Early in pregnancy hCG quantitative levels rise quickly, normally doubling every two to tive days before seven weeks. But there is wide variation in this range. Faster rates have an association (weak) with twins or multiple gestations, and possibly with molar pregnancy, though again these are not absolute, and not used to make either diagnosis. US will be the better choice.