If i had sex three weeks after my period (i am still breastfeeding my 7.5 month old child) and a day after i got blood spotting. could i still be preg?

Yes: Hi. Breastfeeding only helps prevent pregnancy by preventing cycling. If you're back to having periods while breastfeeding, you must assume you're fertile. Spotting a day after sex does NOT exclude pregnancy. If you miss your expected period, get a pregnancy test. If you want pregnancy, OK, but if you don't, might I suggest an acceptable form of birth control. Good luck!