Welcome to Pentax Forums, Digimom,
Even if you did have an aperture ring on your lens, and it set to 22 I believe that your camera is design hold it wide open.
So that isn't your problem
It not being on all the way is one possibility, however I would imagine that was the first thing you tried.
So I'm going to say, it sounds like sticking aperture blades. Which you can't fix yourself
So, if the lens is on warranty you might want to look into getting it repaired. I don't know how good or bad Tamron are with their warranty repair. I hope that it doesn't take too long for them.
If it isn't under warranty there is some fellow in the US, whom several board members recommend. Although I don't remember his name, someone may bring it up.
Here in Canada I know that Gentec International repairs Sigma lenses, however I don't know if they'll touch anything else. They did a great job repairing one of my Sigma lenses