Right now you need to consider what your March period was like. If it was normal, you would be looking at a post-march-period-conception. If it was light (and if March period was early!) the March period may have actually been implantation bleeding.
Also consider when your April Period would have been due, and count back 2 weeks. This is when you would have ovulated (assuming not impregnated in March), and will help you determine likely of new bf being the father.
If you had sex with your ex in March, and had your period after that you did not get pregnant until April. You will have an u/s to date the pregnancy, and that will give you a pretty accurate conception date, but the only way to really know 100% is through a paternity test.
get an hcg test done. the levels will indicate how far along. also most of the time, conception is about 2weeks after your last monthly period. if you had sex before your period in march and didnt get it this month, i would suspect ex boyfriend. but how was your period in march? regular? heavy? light? best way is with an ultrasound, or bloodwork. but i understand you probably dont want to start all the testing before you know whos it is. good luck, and god bless.