Thank you for your question! If your period is between 30-31 days then you're period was due on July 27-28, I would wait at least another 4-5 days prior to taking a home pregnancy test.
Many times periods can be late due to stress, illness, medications, etc. Your immunization shots, however, will not stop you from getting a period. Remember that any time you have sex, whether or not there's ejaculation, you could potentially get pregnant due to pre-ejaculation if not protected.
Good Luck and keep us posted,