What are the chances of an incorrect pregnancy test? ?

I've been bugging my friend to take a pregnancy test because she hasn't had her periods since December, but she said they have been very irregular before then, she went to the doctors in February and the test her then and it came up negative anyway. I made her do a pregnancy test even though she was certain she wasn't pregnant. The test came out positive. My friend is still convinced that it's incorrect but I'm not so sure. I've noticed her belly has been alot rounder lately but not huge anything almost like she's bloated but it always looks bigger at night & no one else would notice. She has put on 2 kgs in the last 2 weeks and her look alot bigger. We worked out that If she is pregnant the baby would be AT LEAST 16 weeks old being that the last time she had was the 18th March. Also her s were leaking some whitish stuff a couple of weeks ago. Is it possible not to show much during pregnancy? Also because she hasn't known we have had numerous binge drinking nights! Will this affect the baby? Do you think she is pregnant & in denial? She's promised me she will go to the doctors this week even though she is 'pretty sure' she's not.