Suspecting you are not differentiating between internal memory for apps (system memory) and that for general storage. You may still have internal memory free for other stuff, bit not free anymore for apps.
A strategy is to move some apps to your SD card. Got to settings then apps (or Applications). tap on an app to open it, and then if the app is movable to the SD card you will a 'Move to SD' button enable. Move it.