What Causes Brown Spotting With No Regular Period?

hi, ive been on the pill for 5 yrs but the past few months i forget to take them, on the 6th of aug i come on my period and finished a week later, but on the 20th of aug i had to have antibiotics and had unprotected sex on that day, about 4 days later i had a bleed which lasted about aday, then nothing untill i was due on last week but all i got was a light brown discharge for about 3 days, is there a chance i could be pregnant. many thanks