If you're anywhere near Kettering, Ohio, my office (a pro-life crisis pregnancy center) can offer one free ultrasound per client per pregnancy for the purposes of verifying gestational age (how far along you are) and viability (heartbeat). They might also be able to tell you if you are having twins! :) And you don't even have to be in a "crisis" pregnancy necessarily! :) And, previous replies are correct, that the more pregnancies you have, the faster your body responds to pregnancy, the more likely you are to "show" sooner!
God Bless! and Congratulations!