This is a very simple answer -
1. Don't worry before you read the answer.
You had a late period, which is normal (day late is very common) some girls/women can be a few days late (non sexual/
pregnancy related)
Basically the clots were most likley caused by sex. You still had blood from your period, and the force from a penis during sex would cause the blood (still remaining) from your period to "clot" and come out afterwards. It is nothing to worry about -
If symptoms persist, and you have trouble with urination, or if intercourse hurts, then you might need to talk to your doctor.
NOW - For a more "harsh" answer, but still nothing to be too scared about. There is something called
Endometriosis, It occurs when tissue that is the lining of the
uterus (tissue, which is called "endometrium") is found OUTSIDE the uterus.
usually in the abdomen on the ovaries, fallopian tubes, and ligaments that support the uterus; the area between the vagina and rectum; the outer surface of the uterus; and the lining of the
pelvic cavity. Other sites for these endometrial growths may include the bladder, bowel, vagina,
cervix,
vulva, and in abdominal surgical scars. Less commonly they are found in the lung, arm, thigh, and other locations.
Miisplaced tissue develops into growths or 'cuts' which go to the menstrual cycle in the same way that the tissue of the uterine lining does, each month usually in the abdomen on the ovaries, fallopian tubes, and ligaments that support the uterus; the area between the vagina and rectum; the outer surface of the uterus; and the lining of the pelvic cavity. Other sites for these endometrial growths may include the bladder, bowel, vagina, cervix, vulva, and in abdominal surgical scars. Less commonly they are found in the lung, arm, thigh, and other locations.
This misplaced tissue develops into growths or "cuts" which relay to the menstrual cycle in the same way that the tissue of the uterine lining would... each month the tissue builds up, then breaks down into smaller pieces, and splits.... Menstrual blood flows from the uterus, out of your vagina, but the blood and tissue tears from endo growths can't leave the body... This can cause some
internal bleeding, cuts, and menstural problems and can cause pain, problems with getting pregnant, and even bowel problems (with bowel moviments) -
If you have any of those symptoms in the SECOND part of the answer, SEE YOUR DOCTOR. Otherwise, I think you just have some "leftover" if you will, and the sex probably caused minor cuts to the inside of your vagina, especially since during your cycle, it becomes more sensitive...
I would not worry if the symptoms pass after a week, but always seek a doctor that you trust and know before "guessing".
Good luck!