There is no chance for the pregnancy
because of two reasons
1... Your periods started on 19th July and continued for 6 to 7 days which means you can't get pregnant on 28th July.
2... Sex was protected sex.
Even if you masturbated one day earlier,it should not cause any problem to you.
Wait for some time,you should be able to have periods.
Get your pregnancy test if periods are delayed by more than a week.
Periods may get missed or delayed due to several reasons other than pregnancy.