my girlfriend missed her period and we are scared

my girlfriend and i had sex about 3 weeks ago...she usually has an irregular period, but this time it was just weird....she told me that she had missed her period for the month, so i bought her the first response pregnancy test, which came up negative...a coule of days later, she had her period, or so we thought...she only bled for 2 days? what does that mean? she hasnt bled since then, and it has been like 3-4 days...she has also been having headaches, which i thought were from stress of thinking she might be pregnant...if anyone can help us out it would be greatly appreciated...and she is going to take another test soon...i will inform on the results...