After you file

My filing status is single, I don't have a dependent as I don't have any kids. The only stimulus checks I had received were the two sent in 2020, I never received a third one. The first one received was $1,200 and the second one I received was for $600, both in 2020. I did received a letter from the IRS about the changes but im left even more confused, I don't understand why I received way less than what I was approved for :(