Any help would be appreciated. Dear Virgil, At 18 months old it is very hard to tell what is going on. At this age it is normal for a toddler to be extremely happy one moment and hysterically crying the next. However, if her mother had a psychological condition, it very well may have passed on to her daughter. Mood swings in and of itself is not a psychological condition.
People have nine inborn temperament traits, one is intensity.
If this is your granddaughter, there could be nothing wrong with her - it is just her temperament. You should expect strong reactions from her and develop a plan to help her express it appropriately or diffuse it. I would stay away from medication at this age unless absolutely necessary. You probably should get your granddaughter checked by a physician - it can't hurt. But my gut feeling is that you cannot make a diagnosis at this young age.