is it possible to get pregnant during your menstrual period

Ok here is my situation...I got my period December 13 of 2011(which last 5 days) and my husband and i had unprotected sex on the 11th and the 16th.My husband left me on the 18th of that same month and a week later i slept with someone else.(12/22/2011).On 1/1/2012 I had what I think was implantation bleeding which was confusing to me since my next period wasn't due until mid January and i had just had a period.So I waited to see if I would get my period anyway on the 13th.I didn't get it so i went to the doctors on the 15th to get a pregnancy test and it was negative but i still had a feeling I was pregnant.on the 22nd I took an at home pregnancy test and it was positive.When I went to the doctor they took the spotting as a period and told me I was 3 weeks pregnant but i disagree since my periods are usually heavy and this was very light blood and only lasted 3 days.I am very confused by this and have a feeling i am further along then what they are saying. I am also unsure who the dad might be since i had unprotected sex with both of them but i am positive it is my husbands.My husband and I gave since reconciled and now I wish I hadn't done what I did with the other person.I don't think this other guy is the dad since he has 4 other kids and they are all boys and my husband and I have 3 girls which will now become our 4th. Please if anyone has any advice please help.