No force is causing you to move forward.
Newton's first law states that an object at rest stays at rest or an object in motion stays in motion unless acted on by an unbalanced force.
The bus was moving forward and comes to a sudden stop. You were inside the bus that was moving forward, but no force has yet acted on you, which is why you moved forward. Some unbalanced force would need to act on you to keep you from moving forward.
Hope this helps and makes sense!
