6max cash game. 2nl. CO has 158 big blinds. CO raises to $0.07. I"m on the button with 128 big blinds and I 3bet $0.21. A short stack in the BB goes all in with $0.32. CO calls. The site will now only allow me to call or fold. I'm not happy about because if I have AA. I'm guessing there is some reason for this? But I want to know why. My thread title explains my ignorance. Please explain. Thank you.

The BB's all-in is less than a minraise so it doesn't re-open the betting.

you raised 14c, his re-raise (the bb's) is only 11c, if you had raised 12c to 19c he would re-raise 13c and you would then be able to re-raise

A minimum legal raise is double the amount of the last raise. In a 1/2 game with a raise to 7, then the raise amount is 5 (raised 5 more than the BB of 2). So the minimum legal raise of that is now to 12. You are of course allowed to raise more, but this is a minimum.

There is an exception to this, and that is when someone goes all in but cannot make the minimum raise. So when you raised to 21, that was a raise of 14. To reraise you again, the next raise must be at least 14 more or a total of 35. Since he does not have 35 it cannot be a legal minimum raise (although he is allowed to go all in). Since there was not a legal raise after you, it's not possible for you to reraise again.

To get back to strategy, this is not necessarily a bad thing. It helps disguise your hand. Possibly a blessing in disguise.