MISSED PERIOS ;NO SYMPTOMS: NEGATIVE P. TEST HELP?

On the 13th and 26th of april had sex, according to the my days app my period should have started the 11th of this month . This app is usually really good and my period always comes the day it is supposed to.I am not ten days late took a preganancy test 3days ago came out negative. Should I still b worried: I've also been feeling lk if I have my period but it stringy clear/yellowish mucos , it not an infection ..so help any ideas ??