While some people will feed you some pseudo-science about probiotics, you don't get a cold because of the bacteria that you do or do not have in your system. Colds are caused by viruses, which are not influenced by bacteria.
Bacteria do influence fungi. Bacteria and fungi are in constant competition. If you kill the bacteria with an antibiotic, the fungi flourish. Yeast is a kind of fungus. You got a yeast infection because the bacteria that normally battle against the yeast got wiped out by the antibiotics, not because your immune system got weaker. Your bacterial flora is re-established by now.
Chalk this up to chance and to recall bias and September.
Chance - sometimes you get two colds in a row after having had strep.
Recall bias - now that you have taken the antibiotic, when you don't feel good, you say, "aha, my immune system is weak", and you remember it.
September - children are back in school together, and people are inside more due to the cooler weather. This combination is the perfect storm for passing around colds, even if you don't have children yourself.