Dear Chimiska,
Your bladder infections might be the natural consequence of aging. As a woman ages the mucous membranes that line the vagina and urethra thin out a bit and can make you a little more vulnerable to infections. You might consider having a chat with your gynecologist or nurse practitioner about this. I also had this problem and my gyn recommended an Estring device. This is an intra-vaginal plastic ring that contains a very small amount of estrogen that leeches out and helps maintain the integrity of the membranes in this area. It stopped the infections and helped ease some other menopause related problems.

The other remedy, as several others have pointed out, might be the saddle you are using. You may have to experiment with either different types or different adjustments. A good bike shop will help you with this.
Good Luck,
La V.