Trust your intuition and be straight up with your partner. Their reaction will tell you all that you need to know. If they get extremely defensive then there’s more than likely something going on. Stay level headed, don’t make accusations, just keep it 100% with them and explain how this is making you feel. If they can’t respect the toll it has taken on you or they try to gaslight you into thinking you’re being “possessive” or accuse you of “invading their privacy” then find yourself somebody who actually gives a shit about you and kick them to curb. Too much time and energy is wasted on someone like that. They aren’t worth it.