PLEASE HELP

Hey ladies I am in desperate need of some back up here. My anxiety is through the roof. my SO and I went through a rough patch back in November and I left for a couple weeks during those two weeks i came home once and there was a woman there whom he had been flirtatious and such with in the past...so i made a bad decision after seeing them together. the next day i was distraught and stupidly decided to spend some time with my ex and we started to have sex but it didnt feel right, there was no attraction and it just felt gross so i ended our time. this was before thanksgiving. I got tested and all was well in the neighborhood. i never planned on telling my SO once we had gotten back together because i didnt feel he was entitled to that information and i felt it would only hurt him but he got it out of me. i had a 6 day period from the 27th nov-2nd dec. so fast forward to december we have sex all through my fertile window and i take tests to no avail and im feeling pretty discouraged by this point but then my tww is over and i take another test and its a vvfl. over the next couple days it gets darker and darker until its a full on positive. fast forward to now my SO cannot let it go that i saw my ex and is convinced that there is a posibility that i am not pregnant by him...he is interrogating and pressuring me for more and i have told him all there is, explained that there is no possibility amd recounted my experience with my ex a million times over. I am so stressed out and angry I am shakimg and not sure what else i can do since he jist doesnt want to listen. So please without an haters can you ladies please shed some more light on the fact that this would not be possible?