Not sure how to handle this... help would be comforting

So my girlfriend and I have been in a committed relationship for about 1.5 years, I am 24 and she will shortly be 22. We have been having sex often and 2 months ago she went off the Pill. We started using condoms and this last month we have used condoms every time in the following way: we would start unprotected and after about 5 minutes would put on a condom. Typically that gave us a full 10-20 minutes before I finished with a condom on. I wouldn't pull out but we never had any slip off or break.

My girlfriend is now nearly 2 weeks late and we have taken 4 HPT tests (post-period morning 6, morning 8, evening 9, and morning of day 10). All of these have read negative. I am terrified she is pregnant but she is in denial and claims 4 tests can't be wrong. She hasn't stopped drinking yet and has some work party, her friends B-day, and her B-day all coming up within the next 2 weeks.

I have asked her to be careful about alcohol until she has a blood test but she doesn't want people to guess if she doesn't drink.

Would you think she is pregnant? We aren't ready for a child (please no "you should have waited" answers, they do no good) because we want to enjoy our lives together as young professionals. We had been making plans to get engaged so we are committed but not ready for this.

Any words of wisdom?