Hi Doc.. I Have Something To Ask. I Started Taking

Hi doc.. I have something to ask. I started taking althea on the 13th day on my cycle.,my last period started on nov. 17. Then i'll take althea on nov.30, and i've done on my last pill dec. 20. Until now no period., i didn't feel anything like im pregnant. Is it normal that my period comes delay. Before taking althea i have a regular period. Looking forward for your answers it would help me a lot.. Thank you