Hi, the ultrasound can be more or less accurate depending on how early in the pregnancy you got it. When did you get told that you conceived on February 11? And did the doctor tell you this? When you got your first ultrasound, at what point in pregnancy was it (counted in weeks) and what estimated due date did they give you?
It sounds to me like you are getting the figure of February 9 - 11 for conception from an ultrasound that has about a week's leeway. It sounds pretty likely this pregnancy is from your boyfriend, but after the baby is born I would definitely get a DNA test just to never have to worry again.