Rotation problem

Hy, I've finally done this function, but it is not working as spected. Resulting coordinates are sometimes wrong. Any ideas?

Function GetRotationCords:TDrawingPoint(Angle:Float, Source:TDrawingPoint, Center:TDrawingPoint) 
	Local Radius:Double = Sqr((source.X - center.x) ^ 2 + (source.Y - center.Y) ^ 2) 
	Local NewX:Double = Center.X + radius * Cos(angle) 
	Local NewY:Double = Center.Y + radius * Sin(angle) 
	Return CreateDrawingPoint(newx, newy) 
End Function

'----------
Type TDrawingPoint
	Field X:Float
	Field Y:Float	
	Function Create:TDrawingPoint(X:Float, Y:Float) 
		Local DP:TDrawingPoint = New TDrawingPoint
		dp.X = x
		dp.Y = y
		Return dp
	End Function
End Type

Function CreateDrawingPoint:TDrawingPoint(X:Float, Y:Float) 
	Return TDrawingPoint.Create(x, y) 
End Function

In this function: Angle is the angle to rotate, Source is the non-rotated point(x and Y), and Center is the rotation center. I don't know why but I'm getting some problems when the rotation center Y is not the same as the source Y, very weird. I have no idea what is wrong.