Pixel Shader Constant Buffer alignment - c++

Question

I have a pixel shader buffer
cbuffer InputBuffer {
float fTextureWidth;
float fTextureHeight;
float fTimeStep;
float padding; //padding to align to 16 bytes
};
That corresponds to a struct in my shader class:
struct InputBuffer {
float fTextureWidth;
float fTextureHeight;
float fTimeStep;
float padding;
};
Which looks all fine, since its 16-byte aligned. But when I render I get this warning:
The size of the Constant Buffer at slot 0 of the Pixel Shader unit is too small (16 bytes provided, 32 bytes, at least, expected). This is OK, as out-of-bounds reads are defined to return 0. It is also possible the developer knows the missing data will not be used anyway. This is only a problem if the developer actually intended to bind a sufficiently large Constant Buffer for what the shader expects. [ EXECUTION WARNING #351: DEVICE_DRAW_CONSTANT_BUFFER_TOO_SMALL]
Why would the buffer expect 32 bytes? I thought 16 was the minimum and mine clearly is?
EDIT 1: I've added the output of D3DReflect and my shader code.
After running D3DReflect on the pixel shader and retrieving a ID3D11ShaderReflectionConstantBuffer pointer to the constant buffer I ran the GetDesc method on the buffer and got what I expected:
Type: D3D_CT_BUFFER(0)
Variables: 4
Size: 16
Flags: 0
As requested, here is the pixel shader code:
/////////////
// GLOBALS //
/////////////
Texture2D shaderTextures[2]; // 0 is the velocity field texture and 1 is the field that is to be advected
SamplerState SampleType;
/////////////
// BUFFERS //
/////////////
cbuffer InputBuffer {
float fTextureWidth;
float fTextureHeight;
float fTimeStep;
float fDissipation;
};
//////////////
// TYPEDEFS //
//////////////
struct PixelInputType {
float4 position : SV_POSITION;
float2 tex : TEXCOORD0;
};
// Pixel Shader
float4 AdvectionPixelShader(PixelInputType input) : SV_TARGET {
float2 u = shaderTextures[0].Sample(SampleType, input.tex).xy; // velocity
float2 rdFactors = float2(1.0f/fTextureWidth,1.0f/fTextureHeight);
float2 prevPos = float2(input.tex.x - (fTimeStep*rdFactors.x*u.x),input.tex.y - (fTimeStep*rdFactors.y*u.y) );
float2 final = shaderTextures[1].Sample(SampleType,prevPos).xy;
return float4(final,0.0f,1.0f)*fDissipation;
}