It's the Murphy's Law of Interviewing: once you accept a job at one facility, you will suddenly be besieged with other job interviews and/or offers
To be honest, as a new grad you do not really have the luxury of holding out for something better: new grad spots are so few in number and have many applicants for each spot. Plus the fact that one facility extended you a job offer says nothing about your chances of getting another offer somewhere else. Sure, you may be one of the fortunate few to be juggling multiple offers...in reality, most new grads have one or none.
While I'm not saying to jump at the first job offer you get, think long and hard before you decide to turn it down. In a dream world, we'd all start in our dream job. The reality is kinder if you look at this first job as the first step to your dream job. And more importantly, in this market the next new grad job interview/offer may be long in coming.
Good luck with the job hunt!