Some Antibiotics do something to prevent birthcontrol pills from being effective, so can get pregnant even on the pill. If you are on BC pill, always ask the pharmacist about side effects of medications.
Of course antibiotics can mess up the normal "flora" of the g-i tract and make you more susceptible to yeast. Yeast can be anywhere, it loves warm damp places to grow. Mouth, gut, vaginal, perineal area, skin folds, etc. It can also get into the blood stream and fungal infections are very difficult to treat. Hormones, steroids and sugar can also be factors.
Look into high quality nutritional supplements to improve your immune system.