Half of our roommates affected, but can’t figure out with what. No telltale signs

I am in college and me and three others live in an apartment. Myself and one other roommate are experiencing bites every night, mostly at ankles and lower legs. She’s had some on her arms and back but mostly legs and ankles and i’ve only had them on my legs and ankles. We have stripped our beds (I have twice) and washed and dried everything on the hottest settings. We haven’t seen any signs (no blood, no bugs, no eggs, nothing). Our bites are kind of sporadic so we were thinking fleas, but another one of our roommates and her cat don’t have bites or fleas. We are freaking out and feel like we are going crazy because we can’t figure out what’s going on and they won’t go away. if you have any advice, please help.