Thank you for posting your query at DoctorSpring.com
Yes, the breast tenderness is hormonal.
The transition from Mirena to the pill can cause missed periods, irregular spotting and bleeding too.
You should not worry about a pregnancy as your home tests are consistently negative and you have been careful enough.
Just continue with the pill pack and wait for a few days for your menstrual pattern to stabilize.
All the best
Please feel free to discuss further.