I have bleeding during sex, mainly during the first round. Afterwards it totally stops and there is no sign. Had a half a day period on the day my period was expected then nothing, totally nothing no sign of blood anywhere

Are you ready for the sex, prior to starting actual intercourse? Is there enough and proper foreplay for you to be lubricated properly?

Are you using protection? Note that pullout/rhythm is not birth control.

Usually bleeding during/after sex is a sign of a lack of lubrication or that the sex was a little too rough. The tissue of your vaginal canal and surrounding area is delicate and can be torn if not proper lubricated. It is a common problem. Maybe make sure that your partner, if using anything other then his penis, is gentle and makes sure that it is without sharp edges. IE, trim his nails or what not.

Reasons of bleeding during sex:

1. Ectropion:
It is due to the hormonal effect of oral contraceptives which increases the chances of the cervical canal's delicate lining to grow out to the cervix's surface.

2. Improper lubrication
If the amount lubricated is insufficient then it becomes tougher for the genitals to bear the pressure. I agree withi what CravenMorhead informed. Consistent sex with improper lubrication cause more troubles to you. So make sure about such things. Also if he is new to you or bigger than you, then it tears vaginal canal and the surrounding a little.

3. It can be a sign of pregnancy? Yes, not normal but perhaps it can be.

If this happens consistently then you must pause it until you consult your doctor. Consult as soon as possible.