Ugh, worst feeling.
What were you storing it in? That stuff acts like a sponge if it's not sealed up tight. I keep mine in the original wrapper until the last second and still get nervous. A dehumidifier in the room might be your only fix if the air's bad. Total waste of good paper, sorry man.
Sometimes even sealed containers fail if there's moisture trapped inside from the start. @alice_king66's plastic bins idea is smart, but I'd toss in a few silica gel packs to be safe. Paper can wick humidity from the air left in the bin when you close it. I learned that the hard way with some watercolor paper that still curled up. Now I squeeze out extra air from the bag before sealing the bin lid. Might sound over the top, but it works.