What you are describing is almost certainly a phishing email, a scammer attempt to steal login and account information. If you check your email with Comcast's Xfinity Connect webmail ( https://connect.xfinity.com/ ), look for the XFINITY Verified Email logo next to any message that claims to be from Comcast/Xfinity. Note that anyone can insert an XFINITY Verified image in the body of an email, so it's important to note not only the presence of that image, but that it is in the correct locations:
See https://www.xfinity.com/support/articles/comcast-verified-email for examples of valid logo placement. A logo in the body of the email does not count.
If the logo is missing when you view the message in webmail, the message is not from Comcast. Unfortunately, AFAIK the logo is not present when you view one of their messages in an email app or program, not in even Comcast's own email app. It's only visible when you use their web email site.
Samples of recent phishing messages can be found at https://internetsecurity.xfinity.com/help/alerts/ under "Top Phishing Scams".
To report a scam email to Comcast, follow the "Report Spam and Phishing Emails" link on https://internetsecurity.xfinity.com/help/report-abuse/ and scroll down to the "Report Phishing Emails" section.