1.) It could be "polestagger." If you play in north america, this factor is still in effect. Some polearms can make you "stagger" a bit longer than usual. It has a % chance of happening, and its pretty rare, so it shouldn't be happening all the time like you describe, and it only works if you get hit with a polearm, and only on NA servers.
2.) someone else could be hitting you right after you get hit. If you get hit with a weapon, and then hit with an arrow, or kicked, you will stagger from both/all hits. This will make you vulnerable for longer.
3.) You are probably not reacting fast enough. Whenever you are hit, you CAN block the next hit, but you have to be fast. Also, in the melee confusion and panic from being struck, you may be blocking the wrong way, or whatever. Usually its easier to hit someone right after they've been struck because they panic a bit and sometimes block the wrong way.
Just try to block the next hit as soon as you're hit and you should be ok.