A calender can't tell you when you ovulate - it can guess but it's not 100% accurate. Our bodies change each cycle and hormone levels fluctuate and so you don't always ovulate on the same date each month.
With regards to whether to mark this as cycle day 1, I would wait and see what the bleeding does. If it stops then it's most likely just caused by the sex. It has to be an actual flow (light, medium or heavy - not spotting) to count as the first day of your period.
When I started the calendar to see when I ovulate it had asked me when was the date of the beginning of your last period, I put Mar. 19th and since April 20th it said 1 day till your period so yesterday I was suppose to get my period but that's when I had sex and the little bit a blood showed and then again today the calendar said 1 day till your period and today after having sex it was more blood but it was light and now it was on his penis not like yesterday..so I think the calendar probably wants me to input the day I start my period..but he too probably knocked mines a little to early idk, I'm going to have to wait and see..
If it stops you may juss be spotting but if it continues How a normal cycle would I would mark it as the start day of my period. I've had my knocked on a few days early because of sex before