here we go

i have been on the combination birth control pill marvelon 28 for 3 years my cycle was always 28 days always started on a thurs always ended on sunday. sept 23/07 my doctor took me off the pill. I had my period on sept 27 and it lasted 6 days. I skiped my period in october and november and got my period december 3/07 and it lasted 6 days. On december 26/07 i had protected sex with a condom what is are my chances of pregnancy like since i have no idea when my next period is due or when my ovulation is due, I really want to go back on the pill do I waite till I get my period before I go back on it