You should introduce her to swimming gradually. Some dogs can actually develop a fear of the water if they are rushed into it. (Not that I'm accusing you of rushing your dog

). Does she look more like she is exerting herself? Is she strong and get enough exercise?
A good suggestion would be to use any toys that she might have. You can play catch by throwing the toy and having her swim and retrieve it and bring it back.
Does it look like she can't support herself too well? Sometimes dogs have problems keeping their hindend afloat. If this is a problem, you can place your hand on her belly near her back for support until she gets accustomed to keeping herself afloat.