I've Had A Head Injury, 3yrs Ago! At Times I

I've had a head injury, 3yrs ago! At times I may get into, crying spells! Nothing has to be wrong, I just get a spirit sadness, not long term sadness. I try to think of happy feelings, but during this time, it's easily to be in touch, with this feeling of sadness, which at that point leafs to crying. Can this be chemical imbalance?