It fits in the time period! Fort Donelson is a little late for it, but it could have conceivably been there.
What you have looks to be a blacksmith made D-Guard Bowie knife. They were popular early in the war with Southerners heading off to fight for the Confederacy. I did say knife, a lot of D-Guards tended to be more short sword than Bowie knife but the name stuck.
Oh and welcome to CivilWarTalk from Texas and the Book Review Forum!