5 weeks, 3rd pregnancy. Already showing!?!!

I feel like I'm going crazy, I didn't show for months with my previous pregnancies. I'm only just 5 weeks along and my pants are all feeling souncomfortable  and the flat stomach I had a couple of weeks ago is gone! I feel like I look about 4 months pregnant! I have been eating healthy, walking and working a lot. I know it's not baby because he/she is still so small. What is happening!?! Has this happened to anyone else?