...missed period

I am 18 and have been with the same guy for three years now ..I ended up being pregnant at 17 and having my son in september shortly after I turned 18. We have been taking precaution when having sex..using condoms and birth control...ive always had a verry normal period and usually keep track of it.. I gor this app because my son is only three months old and I dont want a other baby for two years or so....but the month of december I had no period..im extremely sxared and dont know if it's even a possibility of veing pregnant..I need advice