Recently i had sex. we used a condom and he pulled out while wearing it. i have missed my period. taken 2 tests, both were negative. should i worry?

Many normal women will occasionally skip a period or two. When condoms are used properly they are effective in reducing pregnancy risk. That means applying a properly sized condom that is well within its manufactures "use no later than " dates and before any intercourse. After 60-90 days OBGYN's often prescribe hormonals to trigger a delayed menses.

Menstrual irregularities are not uncommon and anovulatory cycle is a common cause. If you miss more than two periods it would be time to see your doctor. If you do not wish to be pregnant, use contraception all the time. You may consider implanted contraceptive or an IUD. Consult this site. http://www.nichd.nih.gov/health/topics/menstruation/conditioninfo/Pages/causes.aspx

Properly used condoms, specially those with spermicide, are very effective in preventing unwanted pregnancy. Not sure when you had last relation, but it usually takes about 2 weeks or so from "date of conception" until urine pregnancy test to turn positive. If not positive after this period, then pregnancy is not likely. Unfortunately, this is not 100% (but close). Good luck.