some girl told you that you were hitting her bladder? BWA HA HA! Unless your weiner is 18 inches or so, I don't think so.
Now, it is possible for a very long man to bump the cervix, but you just switch positions and it is fine. Those men are like in the 99th percentile for size, though.
Generally, when a woman tells you "Oh, it is so big it hurts" what she really means is "I better say it is so big it hurts so he doesn't think I'm loose."