Plan B failed. Pregnant. Who's Baby is it

Don't judge me.. But I am way confused and I need help..
I had unprotected sex with my ex boyfriend on August 13th. My period came around the 1st of August, either right before or after, I cannot remember exactly, but it's always at the very end of the month or very beginning.
I took a Plan B 48 hours later.
Then on a very drunk bad night a week later, I had sex with someone else on the 19th. He said he withdrew during ejaculation, but I am not positive about that.
I know that when I had sex with my ex on the 13th, I was more likely to be ovulating..
By the way, my period came again on August 30th, which is when it would have come normally. But, I took a pregnancy test last week and I am definitely pregnant. Who's baby is it? I'm flipping out. Can someone PLEASE give me some insight, I'm losing my mind.
The only solution I have thought of is when I get an ultrasound and find out how many weeks I really am pregnant, and count back the weeks to see who it was, but is that even accurate?
Please, please, please, Thanks.

[wanabb]

Your period on aug 30th could have been break through bleeding making it your ex's but it really depends on when you ovulated and if you had unprotected sex with anyone after the 30th.  Get an ultrasound, this early they can tell the baby's age close to the day.  Good luck.

[wanabb]

Your period on aug 30th could have been break through bleeding making it your ex's but it really depends on when you ovulated and if you had unprotected sex with anyone after the 30th.  Get an ultrasound, this early they can tell the baby's age close to the day.  Good luck.

[JennyB0125]

First Day of Last Menstrual Period:  August 1, 2011
Probable Date of Ovulation:  August 15, 2011
Possible Dates of Conception:  August 11 to August 19, 2011
Due Date:  May 7, 2012 (40 weeks)


First Day of Last Menstrual Period:  August 30, 2011
Probable Date of Ovulation:  September 13, 2011
Possible Dates of Conception:  September 9 to September 17, 2011
Due Date:  June 5, 2012 (40 weeks)

[bgeek167]

Thank you guys, I feel a little better now.