Could Knots On Mastoid Bone Be Related To Hormones Or Menstruation?

Hello I have a 15 yr old daughter that has two (I am assuming lymph nodes) knots behind her right ear located on her mastoid bone that are painful. These places come and go. The last time it was noticable was in July of this year. She as seen her doctor that was not concerned. This may sound crazy but we have noticed these places appear about a week before her period. She also has battled with canker sores in her mouth on and off for several years. She is very active and in good health otherwise. Do you think in anyway this could be related to her menstural cycle / hormones? We have also noticed when the knots appear it is when she has been under more stress than normal and also when she has the canker sores and some acne before starting her period.