My plants are about 6 weeks old and are in 6 inch pots with miricale grow potting soil, the bottom branches started to wilt and got mushy. They didn't change color they just started to wilt. After awhile the wilted branches shriveled up and fell off. Is this a sign of over watering or are the plants sick? This is my first year growing tomatoes and I am completely lost. Any advice would be greatly appreciated!