Why are my night sports photographs blury?

Ok, so let me start by saying I'm almost a little embarrassed that I'm even asking a photography question considering that I'm a photographer, but until now I have never did night shoots. I have always done landscapes and day shoots.

Recently I have been shooting my Niece's soccer games, and photo's are coming out amazing however now they're going into state championship games which are at night. My pictures are blurring bad, so I'm asking if any sports photographers can give me some tips with the equipment I'm currently using to clean up my night action shots.

I have a Nikon D7000 with Tamron 70-200mm F2.8 currently shutter speed is at 500 since that is what I use during the day to shoot the game. I'm using a monopod to keep from having to deal with camera shake. I also have a high power speed-light but its only good up to 70mm. I was thinking of lowering the shutter speed to account for the low light, but was worried that would create more blur. Any thoughts?

Added (1). "fhotoace" thank you for your imput I'm currently at 1/500th shutter speed, but I'm about to check out your link. All the fields are high school stadiums. "Funky Monkey," my shutter is high the blur is from motion.

Shutter speed, shutter speed, shutter speed.

You have to 1) shoot at shutter speeds of 1/500th second or faster and 2) to do that, you will need better than average field lighting.

All the fields I shoot on are ones that sports TV attends, thus the fields are about a stop to a stop and a half brighter than your typical high school or municipal field.

Here is a shot I took using my camera at ISO 3200 using a 300 mm f/2.8 lens.

While your lens is only about 100 mm shorter, at f/2.8 and ISO 3200 you should get some excellent shots from the sidelines. As you can see in this shot, shooting at the peak of action will allow you to shoot at shutter speeds of 1/500th second with excellent results.

One thing that will help your shooting is to get a good reading of the field and then set your camera to manual and use those settings for the whole game.

The good news is that State championship games are usually held on fields with better lighting and yes, using a monpod is an excellent idea, shoot slower than 1/500th second will capture a lot of subject blur.

There are 3 ways it can be blurry, motion blur from moving the camera too fast while tracking the action, action blur from too slow shutter speed, and just plain out of focus. You really need to determine the cause to determine the solution.

If your blur is from motion, you must increase shutter speed. You may then need to increase your iso to compensate, but the only way to eliminate subject motion blur is with a faster shutter