When you login account using Facebook it synchronize your account with Facebook by using details from Facebook and didn't ask for password. But when you login account from a separate device or PC, it will ask for the password.
See from the Instagram Help Center how to reset/create a new password:
If you can't remember your password, you can reset it through your email address or your Facebook account. To reset your password, first tap Forgot? next to Password on the log in screen.
- To reset through your email address, tap Username or Email, enter your username or the email address you used to create your account and tap search. Choose Send a Password Reset Email.
- To reset through Facebook, tap Reset using Facebook. You may be asked to log into Facebook. You can then enter a new password for the Instagram account that was most recently linked to your Facebook account.
If you can't access the email you registered with and you didn't link your Instagram account to Facebook, we're not able to give you access to this account.
For above activity I would suggest instead of app, go to instagram.com and change password.