Week 1

If you are pregnant congratulation!

However, you usually don't get to know it until Week 2 or 3. Unless you get yourself a blood test.

I didn't know I were pregnant and thought my period is approaching. You will feel extremely tired (bare in mind, you are carrying more than one hence most of your energy will be used up on building the placentas and multiplying the cells). If you feeling pain as if you are having period, that's normal because your uterus is expanding to fit your zygotes and soon to be embryos. 

You do not have fatigue yet during this week, and there will no obvious symptoms to show that you are pregnant neither there is any symptom to show you are pregnant with multiple babies.

If your pregnancy test kit shows negative, you shall do another test the following week using this ClearBlue Digital. I strongly recommend ClearBlue as it is very accurate and no point spending so much money and time on cheap inaccurate pregnancy test kit. The reason why I recommended this particular pregnancy test kit is because the result show is digital and it even tell you how old is your little zygotes. Just like this:

Sorry, I can't get the English version online and this is in German showing 2-3 weeks old.