Why do you have left over antibiotics?
That could be your problem, you didn't finish all of your antibiotics last time and whatever you had has come back, except worse now. This is how germs become resistant to antibiotics, because people don't take antibiotics correctly. You start to feel better so you quit taking them thinking that everything is gone and that you are all better now. They give you a certain amount for a reason and the label clearly states to finish all of it. Just because you are feeling better doesn't mean that whatever infection you had is completely out of your system, and taking the leftovers now isn't going to be enough either.
You should go back to your doctor and tell him that you did not finish your last dosage of antibiotics and that you are sick again.