Yes, antibiotics can cause a delay in your cycle. But so can breastfeeding. Sore nipples usually would only occur because of hormonal fluctuations (usually due to PMS or pregnancy). If you've been breastfeeding for a while then I doubt the sore nipples would be from that since you'd be over the painful side of breastfeeding at this point.
If you suspect you're pregnant, you have to take a test with first morning urine and that will tell you whether you are or not. If not - then you know it's just late either because you are still breastfeeding (which alone makes your cycles a bit irregular) or because of the antibiotics.