Is this a mental health issue?

It sounds mostly like just teenage hormones messing with your mood and emotions. Make sure you don't use caffeine ( like those energy drinks etc.), easy on the junk food and sugary food, and make sure you eat something every 3 to 4 hours to keep your blood sugar level - this affects your moods way more than you realize. Go ahead and talk to that teacher you had last semester, don't feel funny just do it. You may want to talk to your counselor at school too.