Welcome to HCM,
Thanks for trusting Healthcaremagic and posting your query.
In a regular cycle of 28 days, ovulation occurs on 14th day. Ovum takes time while coming down through oviduct
As duration of your menstrual cycle is 26 days, ovulation is expected to occur on or around 12th day.
You have not mentioned for how many days you slept with your ex, a week after your period?
As your period lasts for seven days, and a week after your period means 14 days from first day of period, thus you can become pregnant due to sex with your ex.
Chances of pregnancy
due to sex, 21 days after first day of period, with new boy friend are very very poor.
Hope you found my response to your query informative and helpful.
If you have further concerns in this regard, you can submit those with Healthcaremagic as well.
With good wishes,