Definitely sounds like you declared war to push someone else's claim on the WRE. Other possibility is bug, and if so I apologize, but I'll wager the first option.
Which can still be very useful if the claimant is a relative, someone you can potentially inherit from, or if the current liege has good reason to ruin your day and you need to replace him with someone more amicable to your previous indiscretions/culture/religion.
If you're new to CKII, don't fret: the game isn't as hard to learn as one might think (though its not super easy), there's oodles of guides floating around the net, and the community is mostly apt to help newbies.