Had sex every weekend in March with my Ex. We broke up end of march. Had one night stand with a guy on April 2nd or 3rd. Got positive pregnancy test on April 7th. Got positive urine test at clinic (Very faint) on April 8th. What are the chances it was from the 1 night stand?

Also, first U/S at 7 weeks said Due Date Dec 18th. Dating scan at 12+ weeks said due date of Dec 13th (5 Days earlier).

I know there is no way to know 100% without DNA testing, just want to know who is most likely the father and when I conceived. Also, don't know when my LMP was.