Please make my mind stop going crazy.

Okay so, I just had a baby 4 months ago .... I am now pregnant again . I don't know how far along I am at this point in time but that is kinda what I'm trying to figure out . Now listen carefully, Me and my boyfriend broke up 5 weeks ago (Feb 3rd) for about 3 days and I slept with someone else.... He didn't finish in me, he stopped early and finished up in the bathroom .... Fast forward 2 weeks (Feb 18th) I get a positive pregnancy test. Like VERY POSITIVE . Mind you, I wasn't really keeping track of my period because like I said, i just had a baby. Me and my boyfriend were very sexually active ... Before that time .... And honestly we were careful as well, we slipped a few times but, is what it is . I just want to make sure there is no possibility of it being with the guy I had a one night thing with. Lmk what y'all are thinking.

Ps- My sister in-law is 5-6 weeks pregnant and I had a positive test before she did .