My turtle is biting the other!

Question
I have two turtles, 1 red eared slider about 5 inches long, 2 years old. and an western painted turtle about 3-4 inches in length same age. They have been put in the same tank since I got them when they were babies. I just noticed today that the painted turtle is snapping at the red eared slider. The slider has little bite marks on her neck where she had been snapped. I noticed once that the slider was doing the mateing dance, but i am not certain of their sexes yet because they are still too young.  The turtles are in a 20 gallon tank and i do plan on getting a bigger tank very soon, but i was just wondering why this is happening all of a sudden. Should i be worried?

Answer
You are dealing with the possibility of a few things. Territory, adjustment
maturing
mating

If you should have a femalle that is gravid then she may not want others in the same area.
they are really to small for them to successfully breed so id tend to go with breeding attempt
but really think territory.  

Id remove both and redesign the tank. then allow them to go back into the tank and see how they are getting along. Let me know if i can help further and Let me know how they do