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about a month ago I started to notice a strange behavior of my turtle, she would swim on surface of the water, open her mouth like 3 or 4 times on the water surface(as if she's eating/drinking water), then tuck her head inside water and blow out bubbles, sometimes she could do this for up to 15 minutes, I don't know if she's just playing or is there something wrong with her? she swims, eats and looks fine....

When a turtle is ill it will have difficulty breathing and will yawn a lot or breath through an open mouth. A healthy turtle will occasionally yawn and may open his/her mouth to bite or hiss if threatened.

As for opening the mouth under water, turtles do this just to get a drink. Also, they may do this in play. For instance, my turtle will bite at the bubbles in the water and the stream of water itself when I'm adding water to his outside pond with the hose. It's just play.

If your turtle is active, has a good appetite, clear eyes, poops normally and doesn't seem to be having any difficulty breathing, then he's probably just a normal healthy turtle amusing himself. Does he have anything in the tank to interest him like river rocks, fake plants, a bridge, etc?

My turtle opens and close her mouth repeatly for 3 or 4 times on the water surface(think of betta fish making bubble), then blow up bubble under water, but no I haven't seen any sign of her having difficulty breathing, she poop, bask and eat normally, I do have rocks and a ping pong ball in the tank with her, I guess she must be bored with what she has...

I finally caught her unusual behavior last night about 3am(since I heard her making noises blowing bubbles in water), she is playing right?
http://s235.photobucket.com/albums/ee16 ... laying.flv

That's pretty unusual. I haven't seen a turtle do that before.

Also, is it just the light or are her eyes somewhat swollen?

How's her appetite these day? Is she as lively as usual or is she rather quiet, wanting to bask a lot?

When she's basking, does she yawn a lot or ever breath with her mouth open?

Looking at the dates on your posts, I'm guessing that this has been going on for about two months. Is that correct?

Yes, as I posted before, she's doing everything just fine, no swollen or any signs of illness, last time I seen her yawn was about 3 weeks ago when she was basking that's about it....She poop, beg, eat, bask, and swim normally, she's been doing it since about 3 or 4 months ago I believe...

My turtle Leo goes through phases where she likes to chase bubbles on the surface too. She blows them out of her mouth and then attacks them.
I'm pretty sure it's not health-related, just some form of entertainment for them.

It looks like she sees a reflection on the water, and tries to eat it to me. It might be a bubble or just playing but, look for shadow or reflection on the water when she does it. It looks as if after trying to bite, and chomping nothing, she has decided that it must be above the water, and is therefore sticking her head out a bit more to catch it. my turtle Spike will spend hours trying to launch herself out of the water if she thinks there might be a bug under the basking platform. usually it's a little clump of algae.

Fred used to do that, although not for that long though, usually only a couple of times. He basically would burp a bubble then attack it...

I thought it was the reflection too but she even do it in the dark(when I turn the light completely off), so I'm pretty sure it's not because she's seeing something...and all the bubbles you see in the tank were all from her blowing out, not from the filter or anything......

this is kinda silly but I used to have a betta fish quite sometimes back but he died by jumping out of his fish tank, ever since after that, I've noticed my turtle starts doing it, and her behavior is very similar to how my betta makes the bubble nest, sometimes I wonder if my betta fish processed my turtle

Ha! maybe that's what happened.

As long as she's eating normally and active and no swollen eyes she's probably fine and has just found a different way to amuse herself.

She almost looks like she's trying to eat something she "sees" in the water---there's no film on it? (Don't know about doing it in the dark.)

She's looking quite good.