There are many instances where taking antibiotics is a legitimate course of treatment.  While taking antibiotics, the helpful organisms in our guts will be killed as well, leading to imbalances in the normal function of the GI system.  In some cases, other "opportunistic" organisms such as yeast can multiply. Quite often, the overgrowth of unwanted organisms in the gut will lead to pain, nausea, or diarrhea.
Probiotics is a general term for supplements containing the beneficial bacteria living in our intestines. Another term is ACIDOPHILUS, because that is a common strain normally found in yogurt. It is recommended to take a probiotic at least two times daily while you are on antibiotics, and for up to two weeks after antibiotic therapy. The probiotics should be taken at least two hours from the dose of antibiotics for maximum effect.
Probiotics can be found in the supplement section of most pharmacies.

I find an excellent way to get natural probiotics is to ingest fermented foods and drinks. There are several beverages that can be purchased in stores like whole foods and Trader Joe’s.  Kombucha is one of my favorites. I even make my own!  Kefir and beet Kvass are two other choices.  In terms of fermented foods, Yogurt, and Kimchi are two popular choices. Just make sure that they have not been pasteurized after the fermentation process. Keep in mind that probiotics don’t recolonize your gut, but rather decrease the incidence of opportunistic infections.
One more great source of probiotics is the soil! Hard to imagine but very true. This is due to the fact that for millions of years we hunted and ate out of doors, exposing ourselves to the germs and organisms in the soil. So get out in the garden or walk in the grass with your shoes off.  
Another great addition to the diet while on antibiotics “bone broth”.  Simply take leftover bones of any sort and slow cook them and strain off the broth, or make soup out of it. Here is an excellent recipe. Bone broth and soup made from bones is rich in glycine and minerals that help to heal and maintain the gut, along with our hair, nails and other connective tissues.
Introducing prebiotics is also a great idea.  Prebiotics are indigestible to us, but feed our innate gut flora.  My personal favorite is potato starch (Bob;s Red Mill is a good brand), which is a resistant starch.  I take two heaping tablespoons several times per week. I dissolve it in cold water and drink it down. Don't use hot water, as this will convert it to digestible starch.  I must warn you that you should not start at this level, as the increase in the gut flora can be immense and overwhelming, with gas and bloating.  It is advisable to start very slowly with a quarter teaspoon dissolved in cold water and slowly ramp up. The side effects that I have notices are better moods, better sleep with vivid dreams.  This may be due to the “gut-brain axis”.  The gut produces more serotonin than the brain, which is associated with happiness.
So there you have it. If you must take antibiotics, keep in mind that they are not a magic bullet, and WILL cause an imbalance of your own gut microflora.  If you do experience any signs of GI distress, call your doctor immediately.