first sex with full protection and my period has stopped?

Hi,
Welcome to ehealthforum.
Condoms do not give you 100% protection. There can be failures resulting in pregnancy especially if pre-cum or semen has come in direct contact with vagina. If you have missed your periods and if you suspect chances of pregnancy, look out for symptoms suggestive of early pregnancy and visit your doctor/gynecologist for thorough examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately).
If pregnancy gets ruled out, and if you are not ready for pregnancy, but intend to remain sexually active, you should consider visiting your gynecologist to get started on suitable regular birth control measures to be protected (after your periods). Be regular with birth control use as instructed. Avoid having unprotected sexual contact if you are not ready for pregnancy. Be in regular monitoring and follow-up with your treating doctor/gynecologist and report any new/abnormal symptoms immediately. Drink plenty of water. Take adequate rest. Maintain healthy diet and lifestyle.
Take care.