Yes, absent a contractual protection from discipline for drinking on your lunch hour, and I doubt that you have such a protection, you can be disciplined, suspended or fired for drinking during lunch. For some jobs there are safety and regulatory issues, but even when there are not an employer can have concerns about an employee who might be returning to work under the influence of alcohol. If your employer prohibits drinking during your lunch break, you had best not drink during your lunch break.
There may be an exception if you are an alcoholic, your alcoholism entitles you to protection under the ADA, you were not intoxicated at work, and your consumption of alcohol did not affect your job performance -- to explore that possibility, speak with an employment lawyer with the details.

