You misunderstood something. $1,400 is the refundable amount of the $2,000 Child Tax Credit.
Because you earned so little, you owed no federal tax to begin with. EITC, whatever amount that was (which I cannot give an accurate number for based on the information you gave) is FULLY refundable. Had you earned substantially more than $12,000, so that you owed $2,000 or more in federal tax, then you would have gotten the full benefit of the $2,000 Child Tax Credit - your tax bill would have $2,000 erased from it. But because you owed no tax anyway, the government let's you keep $1,400 of the $2,000 and the other $600 just disappears.
So, you get $1,400 in refundable CTC and $135 of the tax you paid in from working. That's $1,535.
If your total refund was $3,100, then the other $1,565 should be the amount of the EITC you were given. Again, based on the info you gave, I can't say if $1,565 is accurate or not.