I am 38 years old, and my period is 15 days late. how likely is it that i am pregnant? or is there something else that could be causing this?

Check preg test: It is still possible so, first, check a pregnancy test. If you're negative, then it's likely you had delayed ovulation for that month and it's going to take longer to get your period. Your ovulation can be delayed for many reasons: stress, endocrine problems (thyroid), other medications, weight gain, etc. In addition, your period will change somewhat as you get older.