Same happends to me when I use certain brand saddles. I would usually take a nice cold shower for at least 1 hour. Try switching to Ariat or Dover saddlery. They make very soft saddles.
It could definitely be the saddle this happens to me when I’m riding in a slippery saddle that doesn’t fit me perfectly. If you feel too weak I would take a couple days off and start up slow you might be pushing yourself too much
I think you should take a few days off and go get a message or see a kariprakter because it might be something more
What exactly do you feel ? Just that you are weak in the legs or is it more a problem to use them, like you don't succeed in putting them at the right place ? Maybe you could try with another saddle ? And if you see that you have the same problems, maybe try to have a class with a trainer, or do some exercices to get a bit more muscles in the legs ?