Sun Flower Plant

My sun flower plant took about five to six days to see my first sprouts. I put three seeds in the soil in my cup and  successfully had three sprouts. I kept them outside all day in a lot of sun and watered them once a day. At about three weeks I noticed that they began to grow at a more rapid pace. They went from sprouts to about two inches tall and had about two leaves. I was excited to see there progress. However, at about five weeks one of the plants began to turn yellow and later died. Two plants remained. At about seven weeks, the two plants gained a lot height and I had to transfer them to a bigger cup. After this week, the cold weather got to them because I accidentally left them outside for a weekend.