M.P.
You are the only one who can figure this out. I suggest you keep a food diary to know how much you're actually eating. Are you eating more than before you gained weight? Know that breastfeeding does require more calories so figure that into how many calories you should be eating. The average adult women needs approximately 2000 calories to maintain their weight.
As to a healthy diet, knowing what that is takes some study. Making meal plans is the best way to ensure good diet. Unless you've consciously planned a good diet it's likely your diet has too many carbs, not enough protein or fruits and vegetables.
Are you getting less exercise than you were before you gained weight. If so that will cause weight gain. What medication are you taking and did you start it 6 months ago. Some antidepressants do cause weight gain. My daughter's doctors told her to stop the antidepressant meds when she was pregnant and nursing. Have you checked with the doctor about taking it?
You mentioned the possibility of a medical cause. Talk with your doctor about that.
I've gained 10 lbs in 6 months more than once. When I consistently ate less and did more exercise I lost it. The older I got the harder it was to lose it.