Yes, a credit card issuer can lower your credit limit. This might happen if the company perceives a greater risk of your defaulting, either through information in your credit report or how you have been managing your card account.
The company should send you a notice at least 45 days in advance of the credit limit reduction.
Mail from card issuers often goes unread, or may even get lost. Signing up for text alerts may help you receive and take notice of important information about your account sooner than by mail.