My last period was June24-28& I'm suppose to have my period now July 16-19& it's still not here? What's going on?

Illness, stress, medication, you just are going to be a week or two late < you are just going to miss this month, the above all happens all the time.

Or if you were sexually active you can be pregnant. Since this is when it is still "due" wait one more week and if no period, take a pregnancy test. Do it first thing in the morning with the first urine of the day.

You could try a test now, but it can be a false negative