what should I do?

Ok I dont know what to do when my husband was showering his phone lit up so I checked it I never look at his phone and I saw a message that said goodnight babe I love you and that upset me a lot reading that the number is an out of state number do you think anything is going on? I know I sound stupid asking that but im confused I dont know how long this has been going on im also 18 weeks pregnant with his baby so I dont know what I should do should I leave or is he just talking to her and thats it I dont know what to do with the baby coming and all I dont want to stay with him if he is cheating it hurts to know that some other women is telling him she loves him and is he telling her back I dont know!!! help me figure something out please

Elicia