This is caused by many reasons,the first of which is that you could have the heat is on too high, or the A/C is too low.
But the primary reason is the heat entering through your windows from direct sunlight.
50% of all heat that enters the home, enters through the windows.
What that means is if your windows are older I.E badly insulated than a great deal of heat will enter through them, especially if they face east or west.
The interesting thing is that heat  from sunlight actually is 3  different types of heat emanating from the different forms of light that make up sunlight, which can be broken down like this
3% UV light
43% Visible light
And 54% Infrared Light

What all this stuff means is that 27%  of the total heat that enters your home is caused by infrared heat.

You may be wondering “well that’s just great but how do I fix it?”

Oddly enough its quite simple, you can either replace the windows, which could be extremely expensive and in some cases, prohibited by your landlord if you live in an apartment.

Or you can get Infrared blocking window tint.

There are many varieties, but the first thing you must understand is that if you have double pane windows you cannot install static cling window tint on them if they are Low-E glass, as It could void the warranty of them.

Understanding that, we would recommend some of our higher end window tints as they block 85% heat.