Yours is not a proof coin. The "square" edge on a proof is on the face of the coin, not the side. You can look at example photos of proof coins from Google Images and you'll see the difference. Also, proof coins would have reflective/mirror like finish on the fields. Yours is a regular 1990 Philadelphia cent.
Actually it is on the edge. On a proof the edge will meet the top surface of the rims (obv and rev) at a sharp 90 degree angle with no roll off or bevel between the two surfaces. And it should be like that all the way around the coin.
Yes, my reply was based on the OPs photos. I guessed that they were trying to show the "square" edge looking at the rim (third side).