Birth control question!

Stephanie
So I started using the nuva ring last month right after my period, after 3 weeks you're supposed to take it out and wait for your period to come that week. After your period week you insert a new nuva ring. 
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​We'll I still haven't gotten my period and the week is almost over! 
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​Is it normal to miss a period after you start birth control? 
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​I am nervous because I had unprotected sex after I had the ring in for 3weeks. I also lost 10 lbs this month and changed my diet drastically. Not to mention it's finals next week so I'm a bit stressed! 
​Does anybody have any experiences similar on the nuva ring?  
​I am not trying to conceive, should I be worried?