Magnesium Citrate is what you are after.
I had that same thing myself.
Started 4-6g of Magnesium citrate took 2 nights and cramps was history.
Take the magnesium daily also off the bike days. Magnesium will help to relax the muscles you can experiment/add on “Citrulline Malete” does somewhat the same you can combine the 2 of them.