Me and my gf had wearing condoms on 25 nov.we did test on 11 dec & was negative.shd i ve 2 tak again.?

Me and my gf had wearing condoms on oct 8....it was her last day of periods....she was bleeding for five days after that......we again had on nov25 wearing condoms......we took pregnancy test on 11 dec and found it negative........should i have to tak test again.......is she pregnant........? she usually has irregular periods......whats the reason she didint get periods yet..................?