A mood change this early in the process probably isn't hormonally induced. HRT is a very gradual process that works over weeks and months and years.
Most trans people go through long years of stress and uncertainty and guilt and anxiety trying to figure out their gender issues. When you finally pretty much figure it out and start taking action, it's normal to feel a sense of relief and/or exhilaration. Lots of people just starting transition with HRT report feelings of excitement and anticipation. Like KayXo said, "you're finally on your way."