Understanding Potential Pregnancy Scenarios: Timeline & Implantation Bleeding

Question
I had my periods on the 18/10/2011 then slept with my ex on the 30/10/2011 then had another bleeding on the 11/11/2011 of which on the 12/10/11 I slept with my boyfriend and again on the 25/11/2011 with my boyfriend. My 11/11/11 periods were about 8 days early,could it have been implantation bleeding making me pregnant by my ex? I had ultrasound on the 20/12/11 which was around 5 weeks then again on the 28/01/12 which was around 11 weeks another U/S on the 12/03/12 which was off and around 20 weeks then another on the 17/03/12 which was around 18 weeks.

The midwives measurements for EDD are around 24/07/12 and U/S dates around 12 & 14/08/12. Who is my baby daddy?

Answer
Based on your dates it sounds as if your boyfriend can be the father, but you don't mention what your midwife is measuring your EDD based on.
If the bleeding on 11/11 was your period, then it is very probable that your boyfriend is the father. Because you are not certain that the bleeding was a period, the only way to be certain is with a paternity test.
Best of luck,
Sheva