Protected Sex and different Period

Patient: My boyfriend and I had protected sex on December 22 (he pulled out when he finished). Today (December 23) I started my period but it is different compared to previous ones. This one is lighter in flow and colour (pinkish/bright red). I’m confused as to why this is, did the sex change it or could I possibly be pregnant? Please explain!! Thank you!!

Symptoms: Slight cramps

Doctor: Hello,you are not pregnant for sure if your menses have started today. Secondly it is common to experience scanty or lesser blood flow n the first day of menses following sex a day prior because during sex and organism, prostaglandins and progesterone levels are raised which may delay proper flow by 2-3 days till their levels reduce and a withdrawal bleed can be naturally established. You may experience passage of clots in this cycle from 2nd or 3 rd , but that is normal for this period. The cycle regularity would not be changed , just that there is a likelihood that this period may extend by 2-3 days than your normal period.I hope i have answered your query in detail,wishing you good health,regards