Hello Meesha and welcome.
I think there are 2 possibilities. It's possible that you are pregnant, and this is a miscarriage. A pregnancy test might be negative when it's going to be a miscarriage, because there is not enough hcg to cause a positive test. If that's the case, the bleeding will likely get heavier at some point along with some cramps, and you'll pass it.
I think it's probably more likely that you didn't ovulate this month. That also will cause a late period. And when a woman doesn't ovulate, she can develop all the symptoms of pregnancy due to her estrogen level getting high.
In either case, when some bleeding is going to happen sooner or later, it's common that the bleeding comes during or after sex.
You could wait and see what happens, or you could see your doctor to help figure it out. If it is a miscarriage, a blood test for hcg should be positive. If there is no hcg, then you know it's not a pregnancy and not a miscarriage.
Since your periods are usually regular, I wouldn't be too concerned. Every woman occasionally has a month where she doesn't ovulate, and after the age of 30, it often happens a little more. Best wishes.