Signs and symptoms (or increased signs and symptoms) are not indicators of multiples. I had twins and my pregnancy was no better or worse than a singleton.
The only way to know if you're having twins is to get an u/s done.
Like the above poster said the only way to know is with a ultrasound. I would discuss your concerns with your ob and maybe they will do an ultrasound to ease your mind
A sonogram will confirm whether or not you are carrying twins. I would discuss this with your doctor. The more kids you have the earlier you start to show as far as your "pregnancy bump." So if you really want to know for sure, call your OB/GYN and ask about getting a sonogram. Best of luck and Congratulations!