What Causes A Delayed Menstrual Period?

Hello there at the moment I'm a little concerned about my period? Perhaps I’m worrying too much. My partner and I had unprotected sex as at the time he hadn’t got a condom. That was the 9/9/10. I am now 8days late for my period, and therefore just a bit concerned because I’m always regular. I was wondering if you could please help me shed light on this because I did a pregnancy test today that came out negative. Last month I did take some medication to stop my period called medroxyprogesteron whilst I was on holiday and so I think the delay in my period is mainly due to perhaps this still affecting me? But I finished those tablets on the 19/08/10 Also I came out of hospital on Sunday due to several severe asthma attacks (was in hospital for a week and half) But I didn’t have any irregular medication to normal other than hydrocortisone IV, an injection into my stomach to prevent blood clots (I can’t remember what it was called) and also a magnesium drip. My partner and I have looked at contraceptive tablets and things but I'm on so much medication for my asthma and I’m terrified of injections, we're not sure what else we can do other than condoms. Anyways...Thank you for any help that you can think of that will stop me worrying that I might be pregnant :)