Until you get an ultrasound, you can't know a due date with a lot of certainty, especially when you had a pause in your flow and then it began again (unless this always happens for you. Some women with bicornuate uteruses have that happen).
However, it mostly sounds like the period on March 18 was your official last period before getting pregnant. For that date, here is what the conception calculator says.
First day of last menstrual period: March 18
Probable date of ovulation: April 1
Possible dates of conception: March 28 to April 5
Due date: December 23 (40 weeks)
According to the conception dates above, the two days beginning on the 26th would have been too early to be the legendary "implantation bleeding."
If you decide to count that bleeding (on the 26th) as the first day of your last period, then your dates would be:
First day of last menstrual period: March 26
Probable date of ovulation: April 9
Possible dates of conception: April 5 to 13
Due date: December 31 (40 weeks)