Brownish discharge in the middle of period

Hi I'm reece I'm doing research for my girlfriend, she is 16 years of age n has been sexual active for over a year, she hasn't taken any pills or sex related contraception for over a month now, and we have been having un safe sex, she has had no sexual contact with anyone else except me same applies for my self so there would be no std's involved, she is due her next period in 2-3 weeks and her last period was 2weeks ago, her period before that was over due a week, but this time in the middle of her period she has had a brownish dark discharge, I'm just wondering what it could be, could she be pregnant or is this normal ?