Go to WALMART and buy a pregnancy test
I knew it. Some people have a sense and they know it. Happened to me with both of my boys. There are pregnancy tests now that you can take before your period would normally arrive.
See a doctor. The tests you buy in the stores are supposed to be very accurate as well.
Only way to be sure is to do a fail-proof pregnancy test. I've been a similar situation, only to find out that it was a sickness unrelated to my reproductive organs. My period refused to start, until I got healthier.