Honestly, with your cycle, anything is possible. It could be the illness that you are taking the meds for, or just a delayed cycle. However, if you are on antibiotics and on the birth control pill, the antibiotics could weaken the bc pill. If you've had unprotected sex, you could be pregnant, so take a test if there is a chance.
If not, no need to worry until a second or third cycle is off.
Good luck!