Yes, No, Maybe?

Hello ladies! Need a little help with this. Last time I had my period was Jan. 18th and lasted until the 20th. My period has came on time since having this app and now I'm a little worried because I was suppose to come on this month on the 12th, now it's the 15th. The app then says my period day will be tomorrow which is weird. I feel like I may be late and don't know if I should test or not. My calendar calculations from when I last had my period in Jan to now is 21 days. Should I wait to see what happens between today and tomorrow? Or test?