Can antibiotics affect your period even when you are not on birth control?

Yes it can because you are fighting off an infection and the antibiotics put your Ph levels off. Once you are over your bout of antibiotics (approx. 2 - 3 days after) you'll get back to normal. Meanwhile eat Cultured Yogurt which helps balance the Ph level, settle the tummy and also stop diarrhea if the certain individual reacts to antibiotics in this way.
Yes. They often make your period early, although in some cases they make it late or just not arrive at all