Pregnant or Not?

Hello mga Mommies! I would like to ask, dapat by Jun 12 may period na po ako, but nung 13 nagkaroon ako ng blood kaya akala ko meron na. I am using Menstrual Cup po kasi. And now when I wake up, usually if 2nd day ng regla ko dapat nakakalahati na yung cup but now parang stain lang siya ng dugo. Kumbaga spotting ganon. I had one night stand po kasi sa ka-work ko. As in one time lang, and we don't have any protection. Nagba-base lang po kasi ako sa calendar method, e nung may nangyari po sa amin nasa middle na po ako ng time span na low chance of getting pregnant na po. I am not sure din po if it's because of antibiotics that I am taking. I am planning to start my pills pa naman na. What should I do po?