Swivel is one of the solution! Major question is what kind of reel are you using? Spinning or bait casting? It is normal for line twisting in spinning, but for bait casting, thats wierd!
If you are using spinning reel! You have to review your own technique. Did you keep crank in your fish when it pulling line out? If you did, this is the problem. Every time you crank in when line is out, the spinning arm will twist the line at least 3-4 times for a single crank. You can figure out how many twist you've made to your line.