Especially if your horse has never behaved as she did that day, it's not generally a good idea just to force her to do something - she was obviously trying to tell you there was something wrong & it may have been because you kept pushing & ignorred her that she was so much worse the following day. I wouldn't muck around any more, but get a good vet out to examine her.
As for what is wrong, with so little info, who knows?? Just because you felt nothing abnormal(& warm feet & legs is not nec a problem at all) doesn't mean to say there isn't one, or that you would notice it yourself if there was. 'Tender on gravel' may not be much issue, but it's not necessarily 'nothing much' either, especially if she were forced to work on it when she was obviously uncomfortable. If your horse is on rich pasture &/or she's fat, then yes, she probably is at risk of founder too, whether or not that has got anything to do with her current problem.