Yes you could be pregnant. Condoms are only about 85% effective and only when used properly. If the condom kept slipping off I'd say there's an even better chance you could be pregnant. Don't forget about pre-ejaculation, it can cause pregnancy as well. Of course like Judy said it could also be your period. I'd wait a week and then take a home pregnancy test.
Maybe the light blood was spotting as a sign that your period is coming. If your period is due on the 25th, but began to spot on the 26th, it could just be delayed, because you had intercourse. If in doubt take a pregnancy test. Good luck and either practice safe sex or abstenance to avoid unwanted pregnancies.