Yes, I've had a very similar situation. I was working part-time PRN at one job after graduation and had a great interview at the University. My supervisor at my part-time job had inquired previous to the interview at the University if I was interested in moving into a full-time position and at the time I had said yes. However, she never mentioned it again and I never received a formal offer. The University then offered me a full-time position and I took it, not knowing that the same day my supervisor had mentioned I'd be joining her team full-time in the staff meeting. Though it was slightly embarrassing at the time, I explained to my supervisor that while I was grateful for the opportunity, the University was a job I couldn't pass up and had to decline the full-time position. She took it well, and I even continued to PRN with them for the next two months until working two jobs became too stressful.

I wouldn't worry too much about it until after you've had the interview and you can better determine the likelihood of being offered the job. Honestly, people do this more often than you'd think: start one job, only to be offered a better or more appealing job shortly into their employment, so they leave early-on. It doesn't mean you're a flake or that you're not loyal, it just means that it wasn't the right job right now for you.