Hi doctor My girlfriend and I had sex on...

Possible causes of delayed period :
1. pregnancy ( if you sexually active ) 
2. Hormonal changes ( expected ) 
3. Some medications such as Medroxyprogesterone acetate
4. stress
5. Obesity
6. Thyroid diseases
7. gastic system disease (wheat allergy)
8. ovarian cysts 
9. malnutrition and low weight 
What to do next ?
-  do home pregnancy test and if its positive , consult an OBGYN 
- if you havent been sexually active recently , do the following :
1. consult GYN doctor to do the needed routine medical test and determine the treatment pathways ( if needed ) 
2. in case of hormonal changes , doctor may prescribe medication to correct hormones imbalance such as duphaston , which you should take a pill every 12 hours for 5 days  
3. in case of thriod hormone abnormalties ( usually accompanied with other symtpoms ) , doctor will prescribe medication such as Levothyroxine
4. in case of other chronic disease such as ovarian cyst ( PCOS ) or other disease , doctor may need addvanced examnation to determine the causes and treatment pathway 
5. general advices :
- avoid stress  and have plenty of rest 
- healthy life style including health diet and regular exercise 
Needed medical examnation :
1. physical examnation 
2. Pregnancy test 
3. vaginal culture and urine test
4. productive system ultra sound 
5. hormone levels including productive system hormones and thyroid hormones