Yes, sex can cause bladder infections. If you are really prone to them (as I am) no matter how much you "clean" yourself after sex it doesn't matter. Some women are "closer together" down there and bacteria gets so easily pushed into the wrong places.
You do need to drink lots of water. Drink cranberry juice or orange juice, but not too much. Too much cranberry or orange juice can actually make the infection worse. Stay away from caffeine. Don't take bubble baths, or baths for that matter, and don't wear underwear with dyed cotton right at the crotch. Those are some tips I have picked up over the years.
To the above post, during different times a woman's cycle their system can get off balance. So, if you get a uti or uti like symptoms at a specific time each month but then it goes away, I would not worry about it. At that time of your cycle your body's balance may just be off.