How many times can I take $10,000 into the USA each month?

Ok so, I have money in my american bank account. I send it through western union to my mexican bank account. Then I go buy dollars at an exchange house. I do this to make a couple hundred dollars every time I do this, just as an extra money.

I travel to USA almost every week carrying no more than $4,000 dollars every time. Every time I go into USA I get asked how much money I have on me, I allways tell them with no problem whether I have 2, 3, or 4 thousand.

Yesterday, they took me into revision, asked me a lot of questions and made me fill in a Fincen 105 form which I read that it’s designed to declare more than 10,000 usd. I filled it in for the ammount of $3592. So, the officers never answered my question: How often can I bring $10,000 or less without having to report it? Is it every week?, is it every month? Is if every time I get into the USA regardless if I go in every day?

I felt that they made me fill that form that by law I was not required to do so. Can some one give me an answer? Thank you.