You shouldn't be experiencing foot pain, especially with Flow bindings that don't need to be as tight across the instep as most other bindings. Advice given above about going to store is good, but if you ride with them again, I'd recommend you fit the binding as recommended by Flow and then see if you still do not have any movement. If not, you may be OK; if you do, you might want to look into either adjusting or another size.
For what its worth, I have high arches and always had foot pain because I liked my bindings tight, feeling more responsive. I recently got Flow NXT-FSEs and while i had to force myself not to over-tighten, it took no time at all to get used to them. They will perform optimally without being cranked down on your foot. If they are tight, you restrict the quick rear entry capability because your boot won't come out freely.