I have had mirena for a little over 1 year with no side effects. I haven't had a period the entire time. However, just this past week, I have had severe heartburn and my breasts are soooo tender. Could I be pregnant?

Possible, but it could be other, more likely, reasons as well. Heartburn can be, well just heartburn from something you ate or drank or acid reflux. Breasts will experience changes throughout the month as well and occasional breast pain is not unusual.

Since no birth control is 100% effective, try a pregnancy test.