Nice job
I mean no disrespect, but I can definitely see this is a first attempt at making a game.
My hat is off to you that the game does in fact have multiple levels and therefor play length. But the game play itself is not all that entertaining.
The animation is unfortunately not as good as it could be. I suggest for the healthbar that at the very least it be centered with the player's sprite. Moreso then that I would make the healthbar invisible except for when the player has recently taken damage. As really the health bar is just taking up the screen with its unsightly length. As for the background I suggest finding a way to make it not so obvious that it is looping.
As for the actual gameplay there really isn't any save for the boss fight. The multitude of enemies you face can just be left to their own devices as you can fly under or over most of them. Some you can even fly into as the damage they do to you is quite insignificant. I would suggest making the player unable to proceed without fighting the enemies. It would also be more challenging if the player has less health or the enemies did more damage.
Something else I feel would really add to the game is making control of the player smoother. Such as allowing the player to move diagonally. It would also be more engaging if the player could shoot in more then one direction. Lastly to add variety I suggest either making different types of enemies or giving the player different power-ups throughout the game.
I can't wait to see your next game. ^^