Since you said your period is always around 28 days your next period must be around 28 aug,
Usually very rarely periods will follow a pattern through out life.
Since periods depends on cyclic secretion of hormones through out months and it depends on many factors.
Sudden emotional disturbance, increased stress
, change in physical activity, other hormone changes like thyroid
, metabolic changes etc.
This muscular cramps followed by bleeding after about 1 week may be indication of next period and nothing more.
If you had any unprotected sex and have doubt about pregnancy
it is better to do a HPT.