My snapchat keeps logging me out

I have an iPhone 4s and overtime I click the app, it logs me out. I never log in with snapchat into a different device. What does this mean?

This could mean that your account is locked, it could mean that someone else has your credentials, or it could be the old phone, my iphone 4 used to do that and to fix it, i would just back it up and factory restore it, it still would end up doing it eventually but was fixed for a long time after factory restoration, i recently left apple and went to a new samsung galaxy and never have any problems…