It’s hard to second guess what your ex girlfriend’s intentions may or may not be and since I don’t know the first thing about your relationship with her, I’m not going to try.
Consider this: what if your ex girlfriend is interested in being more than friends with you? This would be a reason for your ex to want to hook up with you sometime soon. However, this is neither good nor bad: it’s just a fact. Remember that you can’t control what other people think or do, so it’s not very useful to worry about what she may or may not be intending, especially since any outcome requires YOUR input also – this means you’re in control of at least half of the outcome.
You mention that you’re still hurt. Take this as a hint that what you should be using your time to heal; in other words, do things that are constructive to getting yourself back together again. Hang out with friends if you’re that way inclined, or spend some quality time with yourself and watch some great movies or read some great books if that’s what makes you tick… just do the things that you enjoy for now and don’t put yourself (and your emotions) on the firing line any time soon.
As for your question: “how to say no to an ex girlfriend”… that’s easy: say ‘no’.
I imagine (and hope) you simply decline the offer. Well, apply the same to your ex girlfriend. If you don’t want to meet up with her, then say so and don’t root around for excuses. ‘No’ will suffice.