A period that is lighter and a few days late, after sex about two weeks before, can still be a normal cycle, but it can also be an early pregnancy change such as implantation bleeding. Since pregnancy is possible here, this needs ruling out rather than assumed either way.
Next Steps
Take a home urine
pregnancy test now. A negative test this early doesn't fully rule it out if your cramps continue, since both an early miscarriage and an ectopic pregnancy need to be checked for then. If the flow doesn't settle into your usual pattern in a day or two, see a gynaecologist.
Health Tips
Go in urgently if the pain gets worse or one-sided, you feel faint, have shoulder-tip pain, fever, or heavy bleeding.