Iam 51 yrs of age and very sexually active, my cycle is normal every month, I had sex at the end of my cycle on the second of this month and my cycle has not started. I took a pregnancy test and was negative, my breast are sore and heavy what should i do

Today is "only" August 28th, so you can choose to wait a few more weeks for your period to begin.

My last cycle was July 29th, it last 4 days and my cycle for august has not come

If you are not having any symptoms that concern you, you may choose to wait a few more weeks for your menstrual period to begin. I am unclear when your last menstrual period began, but a normal range for a cycle is between 21-35 days. It does not sound like it has been more than 35 days since the beginning of your last menstrual cycle, so your period may be delayed for any number of reasons: stress, new medication, recent illness, or other "unknown" reasons. Women in their 50s also may experience peri-menopause or the beginning of menopause, and your periods may become less regular.

Lastly, depending on the timing of when you took the pregnancy test, it may have been too early for an accurate result. If you are expecting your period at the end of the month, you may wait a few days and test again (if you are having unprotected sex, there is a risk of pregnancy).

After checking through all of these factors and conditions (negative pregnancy test after 2 weeks of unprotected sex, over 35 days since last menstrual period), you may choose to call your doctor to ask about checking your hormone levels (or, ask what their protocol is if you begin developing irregular periods). Women absolutely can experience an irregular period (or two!) in their lives, and it does not mean anything is wrong, and it does not necessarily indicate a medical condition. If you are having symptoms, please call your doctor, however!