Ah, well it is a problem, but I'm talking about after-class aches and wanting to have less of them.
Both seiza and backfall ukemi, which means hitting the mat with the top of my foot flat with the rest of my leg, mean for me putting stretching stress on my feet and ankles in ways they're not used to yet. All the ukemi we did in yesterday's class were backfalls, so last night my ankle was sore. It's not this morning, but I tried sitting seiza for a bit and it did make it harder to hold it for long (besides that my carpet isn't as springy as the dojo mat).
It may be a range of motion problem - or it may just be a conditioning problem, in that your feet aren't used to being stretched in that direction. If that's the case, then they should become gradually stronger just through repetition.