Clots come from low platelets, not high. And yes, in some cases they are normal. If they're really big for more than a day or two, then go get checked out just to put your mind at ease. I find that I most often have them at the beginning of my period and then they taper off as it gets toward the end.

PM me if you want more info -- I have had more than my share of experience with this stuff, but I'm too tired right now to type it out again for all the world to see.