Mar,
Here's a couple of facts you need to know.....
All women do not bleed when they first have sex. They only bleed if their hymens (cherry) have not torn. Women's hymen's can tear from bike riding, gymnastics, horse back riding, falling onto a branch while tree climbing, using tampons, and a multitude of things.
Teenage women often naturally miss menstrual periods. Especially if they are under stress. SO missing one period does not mean you are always pregnant.
Bleeding after sex can be from a variety of reasons.
There can be a little bit of blood left in the creases of your vagina- and this blood would be more brown than red.
It can be a little bit of blood that is ready for a menstural period that is coming soon - then it will be bright red and possibly stringy or chunky.
It can be from an injury to the inside of your vagina. If the injury is mild it should stop very soon and the blood will be very bright red. If the bleeding does not stop in 3-4 hours and you are not expecting a menstrual period soon - you need ot go to a doctor.
So my guess is you have nothing to worry about.
I hope you are now using protection. Statistically, over 90% of girls under 16 get pregnant during the first six months they have unprotected sex. For some girls they might be lucky and it is the 20th time they had sex after 5 1/2 months, but for others it will be the 1st time, and for others it will be the 5th time in the 2nd month. Be careful - unless you want to be pregnant.
Susan