Start job searching. Now. If you look for something new while you still at this job, you won't have to check that little box that asks if you've been fired, and you won't have to explain why you've suddenly departed from your first and only nursing job.
If this is your first job after graduation, you're going to need this experience on your resume. Is there anyone at the clinic that would be willing to give you a positive reference? Maybe the physician that gave you a heads-up? They seem to care about you.