can i still be pregnant after using prostinol?

Yes. Although there is a high chance you are not, there is a disclaimer on the box that says that it is only a certain percent effective (sorry I forgot the actual percent).

how soon can i do a pregnancy test to know for sure. my period is due until the may 3

Usually you have to wait until your missed period, but there is a new test out there that claims it can tell you 6 days before you are supposed to start.