Losing hardness during sex?

So recently the past 2 times with my girlfriend I've lost my erections during sex. We've had sex a lot of times before but these are the only times this has happened, she doesn't turn me off or nothing and the foreplay is great. Although the condom I used was a Trojan ultra ribbed which in the past was fine but this time was super tight and numbed my penis out. Also we've had a pregnancy scare for the past week I was kinda thinking about but still could it possibly just be the condom? I don't see any other reason why I can't get hard as we have had sex before without any problem like this.