You can get thrush after a course of antibiotics, because the antibiotics can kill the "good" bacteria and allow the fungus to get the upper hand.
Likewise, steroids, particularly inhaled steroids from asthma medications can cause an overgrowth of fungus. It's CRUCIAL to rinse your mouth well after using steroid inhalers (powder or spray). I'm allergic to anti-fungals, so I'm vigilant about this, because oral thrush would be very hard to deal with if I got it.
The fact that you got this also means that you need to work on your "flora"--beneficial bacterial populations which overpower the fungi that cause thrush--by eating lots of fermented foods (not fermented alchohol!
) like yogurt, sauerkraut, pickles, kimchee, kamboucha, etc. You can also take acidopholous capsules--good ones are found in the refrigerated section of your local health food store.