1. The learning curve in BJJ is long.
2. Beginners tend to Fight using strength vs relaxing and using technique because they don't really know what to do or how to so did properly therefore they fight. Quit fighting and play.
3. Learn to survive first rather than winning. Learn the hierarchy of positions, how to maintain control in those positions, then start learning how to attack.
You are using your body in a different manner and therefore muscles and joints are taxed in a different manner so they may well be sore.
Survive, Position, Control, Execution. Stop fighting, have fun playing Jiu Jitsu, and you will grow much quicker.