Can't get current Textbox value in OnClick

I have an image button that I do an button_click with

protected void ImageButton1_Click(object sender, ImageClickEventArgs e)
{
stringt value1=TextBox1.Text;

I get the value here ok, but only if I don't set it in the pageload part.
TextBox7.Text="a"; // if this is in the pageload, I will always get "a"

I want to be able to change the value on the page, and have it show up in
ImageButton1_Click. Anyone know why this is happening.