Could i be pregnant or is this just gastritis?

so my period started and ended April 27-may 1, i had unprotected sex on the 2nd. so on the 12 i started to get horrible AF like cramps which seem to radiate to my lower back. I also started to spot just lightly. however the spotting gradually got worse and lasted 13 day. i also experienced mood swings and my left nipple was extremely sensitive. I was also constipated and bloated. I went to the ER the day that i was expecting my period and they took a blood and urine sample, they said i was not pregnant however some of my symptoms sounded like a case of gastritis, i was wondering could i actually be pregnant and not properly diagnosed?