Bleeding after sex?

[angel237]

Hi, I sometimes but not always bleed after having sex. I am not sure of the cause however I have recently been tested for stds including chlamydia, gonorrhoea, syphilis and HIV all came back negative. I did these tests for chlamydia and gonorrhoea one week after sex with someone and then again along with the syphilis and HIV test 6 weeks after sex. Had no sex with anyone else in between. so I am sure I am clear of those STDs or could I still have them?
I am on the contraceptive pill DIANE 35 ED
Both times I have bled after sex I have used the pill to skip a period. The first time it happened was when I should have been due for my period but I was still taking the pill (it was the day I would have got it if I stopped a few days before when I was supposed to) this time it has happened after the second month, so I skipped the period and continued taking the pills for a month and then the day I stopped taking it (after skipping for a whole 4 weeks) I bled after sex again. Could it be due to the pill/taking it to skip periods? I don't have any spotting or anything in between, no problem skipping a period, I don't get anything if I continue to take them but when I've had sex it's bled and I just know that on both occasions it's bled I have skipped a period with the pill. So could that be the cause?
I'm so worried I might have a disease or something wrong with me. It's so scary to see blood there after sex.