Okay, So I Am Extremely Concerned, And Was Wondering If

Okay, so I am extremely concerned, and was wondering if you could help! I had sex on the night of April 20th with an ex (possibly after midnight or later, so early morning April 21st), and then on May 3rd with my current boyfriend (and after midnight as well, so morning of May 4th). The date from April, he pulled out, and the date from May, I took a plan B three days after the fact. My periods used to be extremely irregular, but had become pretty regular, give or take a week or so. According to my LMP, the father would be my ex. However, I had an ultrasound done at 5wks & nothing was able to be seen except the sac. I went back a week & a half later, and measured 6wks5days, giving me a due date of Jan. 28th. At 11wks, I had another ultrasound & they changed my due date to Jan. 26th. I thought this baby was for sure my current boyfriend's, but now knowing sperm can live inside the body for 3-5 days makes me scared. Ultrasounds can be off by 5-7 days, can't they? Not to mention, I smoked marijuana before I found out I was pregnant, which could lead to smaller measurements? I'd just like to know who most likely is the baby's father, & if it's possible for either one of them to be. Oh, and at my 19 wk ultrasound (giving me the same due date), a couple of the pics seem to really have my ex boyfriend's profile, but was it too early to tell or could it have been a distortion? & lastly, a few days after having intercourse in April with my ex, I got very sick for a week and a half (I was working out a lot too)