I am not getting periods.its been 15 days.pregtest was -ve.did it 2 days bak.got my last 1 on 19th dec.got sexually active on 23rd dec but used condoms.n its 5th feb today but still not periods.I am 20.dont want to get pregnant.please help me.

I mostly have irregular periods

Then relax, stressing might cause your period being late. If you used condoms CORRECTLY (checked they didn't break, pulled it out holding it from the base etc), then you should be fine. I also tend to get paranoid when I haven't had my period but it's just stress.

There can be lots of reasons for a late period or even missing a period altogether. Common reasons are things like stress or when you've lost a lot of weight. However it can sometimes be more serious. See here for more Missed period - causes and treatment (http://www.womenrepublic.co.uk/health/missed/)