Welcome to HCM,
Your period is delayed by more than 4 days,if you are not using any contraceptive
and had unprotected sex during period of ovulation pregnancy
is fist we need to rule out so please take up urine pregnancy test and further action would depend on result.
If test is positive and you want to continue with it or not consult your Gynaecologist to take care of you.
If negative then you can wait for few days and then see what happens,if you do not get period repeat the test and consult doctor
to confirm or rule out by Beta HCG
Other causes may be hormone changes,stress if any , severe anaemia etc.
Your abdominal pan may be due to Gastritis
or hyper acidity problem which can be easily treated.
Best of Luck.