Well...depending on how much of a regimen you are on with taking your pill (it is 99% effective if taken at the exact same time every day of the cycle)...if you don't take it with that accuracy, then you introduce a factor that is fairly unpredictable. Then there is the whole "properly installed" condom....basically, it is possible to get pregnant with every birth control technique except abstinance...only one person has gotten pregnant on record who was abstinent.
There's ALWAYS a chance, although in this scenario the chance would be less than 1% if you are properly using the birth control and it's the pill. The pill is 99% effective if used properly, plus you're using backup birth control (condoms), too.
There is always a chance you will get pregnant, each time you have sex, no matter what birth control methods you are utilizing. Also know, that drinking alcohol makes the pill not work, so if you drink alcohol, it will make taking the pill pointless. It will have zero effect.
Like @Mississippi said, its important to take the pill at the same time each day- but seeing as you are on it, you should already know this. And you should already have been told that it is only there to reduce the likelihood, and not completely stop it from happenning. And I'm sure you have already heard that condoms break all the time. Make sure to use new ones, and don't use any that have been laying around for a while, or that have been in a heated environment (ie. in the sun, left in a car etc.) They start to disintegrate very early-on, so don't take the risk, if in doubt, just buy some new ones.