I had sex just last night. we used a condom and it didn't break. i checked my cervical mucus and it seems that i was fertile then. will i be preg?

Always possible: Of course as you know anytime you have sex even with protection pregnancy is possible if you are fertile

Possibly: Depending on how it is used, condoms have about 20% failure rate per year of use. Application of a well fitting condom before any intercourse and changing it for any renewed activity improves protection. Any intercourse uncovered increases pregnancy risk.