sex on the third day of my period and today it stopped?

Hi,
Welcome to ehealthforum,
Sometimes having intercourse during your periods/menstrual bleeding can stop the periods (due to sudden hormonal and local changes). You become closer to your fertile days after finishing with your periods. For a regular menstrual cycle of 28-30 days, fertile days would be between day 8-18 taking day 1 as first day of bleeding. Sperms once released inside you, can stay alive for 3-4 days. You can consider taking emergency contraception within 48 hours of the unprotected intercourse to prevent pregnancy if you suspect chances of early ovulation (egg release from ovary).
Wait and watch for your next periods. If pregnancy is suspected, look out for development of any other new symptoms. If you do not get your periods within 10 days of expected period dates, or if you get abnormal periods, or if you develop symptoms suggestive of pregnancy, you should consider visiting your 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 intend to remain sexually active, 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 and repeat use of emergency contraception.
Take care.