I had my menses on 18th of march, slept with my ex on 30th and slept with my current boyfriend on 6th. Now am pregnant but can't tell who is responsible.on the 8th of April,I took a pregnancy test but was negative. In both cases I took an emergency contraceptive.

It's impossible to say who the father would be with dates this close. The only way you will know for sure is to get a DNA test after the birth of the baby.