Had unprotected sex fri-sun. following weekend had light bleeding for a day, but negative pregnancy test 2 days after? my periods regular. any advice?

Use protection! :): Unless you are trying to get pregnant, always, always, always use protection. This is so important, not only to avoid pregnancy, but also reduce your risk of STDs! Keep a calendar and record when you are having your periods. There is smartphone app that you can use to keep track of your periods and it will also show you when you are most likely to get pregnant.